i have a seagate barracuda 7200.11 500GB hard drive, and it's very quiet. hard drives actually have adjustable acoustics levels. my old dell computer had an option in the bios to adjust the hard drive acoustics mode to either silent, balanced, or performance, and the difference between the 3 is huge. on silent, it literally was silent. but you don't need to have this option in your bios for it to work. HDD inspector and im sure several other programs can adjust the acoustics mode from within windows.
The recent seagate and maxtors i picked up were far more clicky and louder than the WD 500 gb 7200 rpm drives i got, but you can't beat the Caviar GP drives, 5400 rpms, theres barely a noise coming from under my desk.
Thanks all! Just what I was looking for too. Been looking for noise level stats on WD7501AALS Caviar Black 750GB as I plan to do some video as well as audio editing on my machine - haven't found it yet.