FWIW, I've done the ram swapping, putting it in one module at a time and it does the same. I know this sounds like a hard problem to diagnose, but I'm also hearing noise coming from this one particular drive.
One other important piece of information is that I suspected this drive had problems some time ago, but wasn't sure, so I used it to store non-vital data. All my linux installs are done on the other 2 drives.
I would install Gsmartcontrol and see if anything is showing up in the SMART logs for the drive. I've had warning of impending drive problems from SMART several times in the past (OS/2 has a useful little monitor which can be run on the desktop and will pop up any time a SMART error is logged - wish I had the same for Linux).
Another option would be to download the evaluation copy of Dfsee (http://www.dfsee.com
), install the Linux version and run a bad block scan on the drive.