I thought of posting about this here after seeing the thread about transcribing 78-RPM records, but didn't want to intrude with it in there...

I got Audacity to cut and paste parts of two audio files together into one in a different order. That worked fine. While I was at it, I saw that the program also has "Noise Removal". At first, that was exciting because a couple of my orchestra/choir CDs have what sounds like mild tape hiss in them. (They are commercial CDs, not burned from tapes, but I don't know how the original recordings were done or what original "master" they might have been pressed from.)

When I tried it out, the files it created played perfectly on my computer in Audacity, Windows Media Player, and iTunes. But when I had iTunes make a CD of the dehissed files to play in my car, what I got in the car had a faint high-pitched squeal. It's higher than any music and might be in the range that a lot of the population can't hear, which could explain why the computer programs seemed to be ignoring it or my home speakers aren't set up to play it. But that only explains what's going on if it's actually always there; I don't know whether the original files I created in Audacity have it, or it somehow got created in the process of making the CD.

What could cause this?