I have a 80GB 8MB cache DiamondMax +9 to replace a 120GXP that went bad. The FDB is very quiet, and even with only one fan running in my whole system, you can only hear a VERY tiny whine if you hold your ear up to the side of the drive. The idle is otherwise silent, although it doesn't seem quite as quiet as the Cuda IV I have in my iMac, but the fans overpower the noise in my PC anyway. The seeks are the loudest I've heard in an IDE drive yet, however, but this is no problem for me as I actually find the seek noise soothing and reassuring - it is sort of a like a crunching of ice in this drive.
Comparatively, the idle noise of my 120GXP was easily audible outside of the case as a nice, muted, yet consistent whine, but with almost inaudible seeks. I also had a WD 800JB before that which a very grating and annoying idle whine, with "thudding" seeks.
So far I'm very satisfied with this drive, and I'm sure that the 8MB cache is a help, if you can get your hands on a drive with one.