Have you tested the hard drives using a different power supply? The barracudas, although quiet, do have an audible whine.

My squeal/whine seems to come from my Enermax 350W PSU... I just put a samsung sp1213n in to replace my dying maxtor and the whine is still there. I thought it had to do with my 9800Pro because it only whines when i use a graphically intensive program. But it was still there with my old GF2. So far I've put a new HDD, new VGA, and new Panaflos... next stop is a new PSU. Lemme know if this helped and what you did to fix it.