I get this really bad crackle and hiss whenever I play any music, but it only happens when the music is quiet. If I'm playing something loud and fast, it doesn't do it. The operating system itself (10.4.9) does it too when it is totally quiet. I can make it go away or reduce it by adjusting the volume with the keyboard, but it comes back after a bit. Seems like a driver issue maybe? I'm really not happy about this since I spent so much money on this computer and it has flaws like this... It doesn't even do it in WinXP, but it does in OS X. (Windows actually runs better on this machine than OS X does... :() I have a recording of it...
http://www.megaupload.com/?d=80V4JT9A (sorry to those that don't like megaupload)
And what it is supposed to sound like: http://youtube.com/watch?v=Q-xIAecXi3Q
something that I have always heard concerning this is if you have any large appliances on the same circuit as your computer (microwave, some TVs, fridge) the voltage fluctuations alone can cause audio distortions.
Another thing that will cause this is dirty power--IE power that inherently has voltage fluctuations, etc.
I would recommend getting one of those $40 UPSs (also serves as a real nice power conditioner as well). Worth a try.
Hmm that is possible, I have a ton of stuff on the same circuit. I know someone with a UPS I could borrow, maybe I'll try that.