I have some audio cassettes that I'm trying to digitize. Live recording, poor quality, lots of noise. I've captured the audio using Audacity, and was able to use its Noise Reduction effect to get rid of a lot of the hiss -- it worked really well, as a matter of fact. The problem with the audio now is that I can hear some sort of electronic gremlins in the background. It's a fairly high-pitched, but constant, stream of noise. Can someone clue me in to what sort of effect or technique I should be looking for to get rid of this noise? I know just enough about this stuff to be dangerous. Maybe not even that much.