They're already pretty quiet. The "loudest" one is the OS drive, and as that one already has slow performance compared to the other ones, the last thing I want to do is slow it down even more. As I said, the two 320GB ones are "louder", but they're not necessarily loud at all.
As for the 320GB drives, they're both the same WD3200AAKS-00VYA0 model, which is the older double platter ones. That might explain why they're slower, period, but it wouldn't explain why one of the same model is much slower than another of the same model.
I actually just realized something, and I'm pretty sure the one is just slower because it's nearly full (~57GB free) versus the faster one, which only has the Windows 7 Beta, and a 2GB partition for the page file for the Windows XP installation on the first, on it (~274GB free). Hard drives are a bit slower as they fill up, but it's often not by a drastic amount, so that much of a difference seems awfully big.