I have been having a lot of trouble lately with noise on my audio in on my 15″ 1.25GHz PowerBook. I use a Griffin iMic for audio-in as the built-in line-in port has never worked and I could never part with my laptop long enough to get it fixed (though, I really should). I figured out that I could switch to the right USB port from the left and eliminate the problem but today both ports began showing the problem. So, I did some googling and found this post on Accelerate Your Mac:
News Archive for March 8th, 2004 in which it was suggested I check out Apple’s Audio/MIDI Setup utility. I did and, sure enough, the USB port had gotten itself set to 1ch-16bit instead of 2ch-16bit. Changing it back fixed the problem.
UPDATE: the culprit was Audacity. When I ran it, I changed some setting so that iMic was mono instead of stereo. This changed the port the iMic was plugged into to 1ch-8bit and left it like that forever more. Setting Audacity to stereo fixes the problem. Of course, my headset has only the one microphone so setting it to mono was a fairly obvious (to me, at least) choice. Now I know better. I generally record in stereo and mix down afterwards anyway.