hey,
Aliasing: This is what happens when you try to record any frequency that is over half the sampling rate. It sounds like very robotic distortion, if that makes sense.
Anyways, I think the most common sources of aliasing are:
1) Cheap sound card with no anti-aliasing filter. I'm not sure this even exists, but it's a possibility.
2) You tried downsampling your audio using a bad algorithm that didn't have an anti-aliasing filter. This is possible, but again, I'm not sure anyone who is going to write a downsampler is going to forget the anti-aliasing filter.
3) You added distortion (an exciter or tube emulation, or even clipping) and the harmonics that are above half the sample rate are now aliasing noise. This is a common problem, and is fixed by either not using that distortion, or only using it if you have a relatively high sampling rate. Combine this with a high shelf eq, and you can have some pretty heavy aliasing happening.
Not sure if this helped much, sorry.