Can I trust that my Geiger counter (GQ GMC 500) is in fact measuring radiation?

I had a look on a Radon map, and you are in an area with very low Radon. That's good. I find it very interesting that you 'feel' this phenomenon. Radiation has no sensation whatsoever. One of the reasons it is dangerous when close to very high sources, without proper protection or detection. The levels you are seeing from your counter are not even worth concerning yourself about. The hot spots are probably NORM in your house construction, or other items/ornaments. Essentially everything is radioactive, to some degree. 

Using the Radiacode with those levels that you are seeing will be pointless. You won't get any good data. Even with weeks to months of samples, the noise would ride over any signal. As NORM has many different isotopes, these will just create counts all over the spectrum and essentially give you a 'background' spectrum. If you purchase a Radiacode purely for +20 cpm background at your home, you will bbe wasting your money. You are going to need 200+ cpm (with your counter) and a couple of weeks to identify anything for certain. 1000cpm and maybe a week. The Radiacode is orders of magnitude less sensitive than what you would require to work out the exact isotopes that are giving you +20 cpm.

Going back to you being able to 'feel' this phenomenon in certain areas of your home, have your considered RF/EMI from electrics/electronics around your home? If you have a lot of electronics going on, and I am including basically everything that runs from mains, you may be better putting your money into an EMF meter such as the GQ EMF-390. I have a couple of LED light bulbs at home and they my EMF meter go absolutely nuts. Some people are sensitive to EMF, some people aren't.

I'll try my best to answer this. Again, these are quite complex. I have a fairly good understanding on how radiation/scintillation and spectroscopy work (I didn't have a clue 9 years ago when I started getting into it), but I do struggle with putting into terms anyone can understand, but I'll try. 

any isotopes that don't produce a curve would be considered part of noise or NORM?

Two things about this - 1, isotopes do not produce a curve, they produce peaks (in a specific energy level - more on this later), and 2, any isotope that doesn't produce a spike above the background is useless and could probably be considered as background. The counts for that particular isotope have to be very much more than the background counts for the same energy level. Actually, a 3rd thing too, noise is not NORM, well, it is sort of. Noise is the affect of NORM, i.e. background. Background is essentially everything contributing to counts that is not an actual source. Essentially NORM and cosmic rays, and the tube's own inherent background counts are all considered background. That is our noise.

can isotopes that are part of a background spectrum be considered as useful data?

Only when their counts are sufficient to peak above the background reading, otherwise, they too contribute to background. As we can't detect any peaks anything additional would be considered background.

It would seem that using the Radiacode this way would produce a background spectrum since the meter is not in one location long enough, but it could still possibly identify certain isotopes?

This really depends on how hot the isotope was, and how close you were to it. I would suggest a minimum of 3 days sat doing nothing in a specific place to create a background reading for that very exact spot. Move it 10 feet and it may be different - this could be higher or lower.) The isotope has to give a quantity of counts very much more than what background is providing for it to stand out.

Is a certain count necessary to identify or say an isotope is present?

Yes, the isotope has to provide many more counts on it's energy level than background is providing for that particular energy level. If you have 20,000 background counts at 1000 keV, 100 counts extra provided by the isotope is not going to give any clear indication something is present. If you have 40,000 counts extra provided by the isotope, it is going to stand out and look like a nice peak.

The Radiacode has identified isotopes like Cobalt-60, Scandium-44 and the Annihiiation line. Is it normal to pick these up?

I'm a little confused by this statement. It sounds like you have a Radiacode already. Is this the case? Or just trying to work things out? But to answer, the Radiacode has identified many of my own isotopes, albeit they are mainly Uranium (U-238) and Thorium (Th-232) it has identified many other isotopes in their decay chains. If I had Cobalt-60 here, it would detect it and the same goes for any isotope within the capable energy range of the Radiacode.

Scandium-44 is in interesting one because it shouldn't generally ever be detected in a home, ever. This isotope isn't common like Uranium or Thorium and is mainly used in medical imaging, such as PET scans and is generally made in Cyclotrons or specific generators. It doesn't occur naturally. It has a half life of 3.97 hours, so if you had any at home, it would be pretty much gone in a few days. It has a simple decay chain and goes from Sc-44 --> Ca-44 by β⁺ decay mode and is then stable. As Sc-44 decays by Beta+ mode, it also emits a gamma ray at 1157 keV, so should be seen on a Radiacode with a peak at 511 keV (annihilation peak) and 1157 keV (the gamma ray that is produced during each beta decay.)

To finally answer your question 'is it normal to pick up these'? No. Not unless you had either Co-60 or Sc-44 close by. These don't constitute an active part of background radiation as they are both man made isotopes. If you are actually finding these isotopes my suggestion would be to consult the environmental authorities right away. It could be a medical isotope source gone AWOL, and this kind of thing has been known to happen in the past. That said though, with a half life of less than 4 hours, it would have decayed already.

I'll try and explain sort of how gamma spectroscopy works. For this I'll try and use sound for my analogy. Hopefully, this will give you a better understanding.

Let's imagine we are in a football field. (This is our 'house'.) Now at one end of the pitch at one goal, we have a microphone. This is connected up to a special spectrum analyzer (like this one https://img.canuckaudiomart.com/uploads/large/1151320-0c4d8c31-teac-eqa220-graphic-equalizer-with-spectrum-analyzer.jpg ). This special spectrum analyzer and instead of each band dropping back to zero when the sound stops, every time the sound is heard it adds to that audio band. So these are the 'counts'. Every time the bass goes, it adds one count to the bass band, and so on. So the microphone is our Radiacode and the spectrum analyzer is our gamma spectrum.

Now, at the opposite end of the field we add one instrument which gives a sound at a particular frequency (or band on the spectrum analyzer), let’s go the centre of the range and say 1kHz. So every time this instrument is heard by our microphone it adds 1 to the counts at 1kHz. This instrument starts playing and the microphone and spectrum analyzer start recording.

At first the instrument is just too far from the microphone and can't be picked up by it at all. However, the microphone is hearing all the noises going on in the background - people talking, wind noise, cars passing, whatever it be, its background. Our special spectrum analyzer is hearing some of these background noises and starts counting and adding to every band in the whole spectrum. The lower bands gain more counts than the higher gain as wind rushes the microphone causing those lower bands to add more counts than the upper bands. This continues for some time, and after a while we look at the display on our special spectrum analyzer and it looks like a curve. Starting higher at the left where the low bands are and getting less and less as it goes into the upper bands, towards the right.

Now the instrument starts to move towards the microphone. After the distance between is some less, the microphone just about starts to pick up the instrument, but is not noticeable above background noise. The volume of the instrument just isn't quite enough to register a count from it. As it moves closer is starts to be heard, but barely. Adding one count every few seconds in between the background noise that is adding 50 counts every few seconds. The background counts are still increasing and now we have a few counts from our instrument at 1 kHz but we just can't see them amongst all the background. As the instrument gets closer to the microphone it starts to be heard more, and we start getting more counts from it at 1 kHz, but we still have too many from the amount of background we have already collected (and is still collecting), but we start to see a tiny little bump at 1 kHz as the instrument is heard more. Then we put the instrument right up to the microphone and it dominates over background. We are now getting 500 counts every few seconds, ten times more than we are getting from background at that 1 kHz band. After a while of the instrument being heard by the microphone 10x more than background we see a distinct peak at 1 kHz and we know that is our instrument and we know the instrument is playing at 1 kHz and looking up our table of 'which instruments play at what frequencies' we look up 1 kHz and find it is a 'Simomaxizer' instrument because this is the only instrument that plays at 1 kHz.

Now change microphone and special spectrum analyzer for a Radiacode 103, or other gamma spectrometer and change the instrument for a radioactive source.

Without the source being stronger than background you are not going to see it, you just aren't. These small numbers 20 cpm, 50, or even 100 cpm are academic in this context. We need a loud instrument, or be very close to a quiet instrument. The Radiacode needs to receive a good signal from the isotope. Without that strong signal it doesn't break through the background, and becomes merely complimentary to the background. Background in gamma spectroscopy is much like background audio noise. It's everywhere and is more in some places than others. The best way to get a good gamma spectrum is to reduce the background as much as possible (shielded chamber) or to increase the gain of the source (put it closer to the Radiacode), or both.

I have just used AI to produce some fictitious spectra based prevalent NORM isotope. When I talk about signal-to-noise ratio, hopefully the following will show that.

 

Can you tell what this is in this spectrum above? I know what it is, and I can see it there, but can you tell just from this spectrum? The signal is low and background is high, so we have a low signal-to-noise ratio, or SNR.

 

Here I have turned up the isotope and turned down the background a bit, it's starting to take shape, Can you tell what it is now, probably if you were experienced enough and knew your tools enough, but it doesn't really shout anything. This would be a medium SNR - some signal, some noise.

 

Now, what about this? This is showing a high SNR - plenty of signal, low noise. This is showing a very clear picture now, so we see the nice peaks, look them up in our library. Pb-212, Tl-208, Bi-214 and another huge peak at 2614 keV (Tl-208 again), we cross reference them and bingo! We have identified Thorium-232. And we knew this because we saw the data and cross referenced it from an isotope library. This is half of the battle.

But where is it, how, why and when? In the simulation I took my Radiacode to the beach and noticed the dose rate (counts) was higher. That's the first spectrum above. I then placed my radiacode on the sand and it increased in dose rate and I started to see something. Could I have nailed it on the 2nd spectrum? Personally, me, yes probably as I knew where I was and that gives the other half of the picture. Simply know what location you are in and the natural geology of the area helps massively. Then I decided to dig down until I hit black sand and I placed the radiacode onto said black sand. Boom! We get our really nice crystal clear spectrum (last picture). The signal is so much more than the noise it creates a perfect picture.

Now, and finally (if you got this far through, well done!) the counts you are experiencing, the signal-to-noise ratio you will be experiencing with be something between the first and second spectrums, and probably more akin to the first spectrum. Without being able to get a good signal from a particular isotope, the background will muddy the water. There are only two ways to make the signal better - 1, reduce background and 2, amplify the signal. Reducing background, when you are measuring an isotope in background is impossible, so we rule that out. The 2nd, amplify the source can't be done as radiation can not be amplified. The best thing we can ever do in this case is to move the Radiacode closer to the source. If you can't get closer to the source you will never get a good spectrum, period. If you can move it closer and get a discernible increase in counts over background, you may get something if you leave it in the same spot, powered on and sampling for days, weeks, maybe months, then you may see a little bump in the background curve and maybe you can use that to identify the source. And even then, once you have identified it and roughly where it is, what do you do about it? If this is an old house and maybe some old brick or stone has a hot spot, are you going to start pulling your house to bits because of this? It's a tough one, really.

Since I'm 'feeling' the effect of what my meter is picking up

This really, really puzzles me. You can not 'feel' radiation. No human can. If you are 'feeling' something, then it is not radiation, and if your GMC Geiger counter is increasing it's counts in this area(s) then it can only be interference of some sort. It can't be X-Rays either as they can not be felt by a human. I actually have a feeling that if you had a Radiacode (do you?) then it may not increase at all where the Geiger counter does. They are two very different bits of kit and both work on very different fundamentals. The Radiacode should receive interference much less than a Geiger counter simply due to the methods used in detecting said radiation.

I honestly have no answer for what you are feeling, nor why your GMC is picking something up. If this were me, the test I would be doing right now is this: Turn everything off in your house, everything. Switch the main switch on the electric coming into your house. Repeat the test again. Do you still feel it? Does your counter still pick up additional counts. If no, then you need to be looking at electrics and the electronics you have running on it. If the effect is still there, then you need to start looking at your environment - neighbours, close factories, antenna masts, 4G/5G mobile antenna masts, any powerful electrics/electronics going on close by, anything really. Have you used your EMF-390 in these areas where you feel this phenomenon?

(I haven't checked this for grammar, only spelling. I'm not going to either as it took too long to write as it is!)