The exact model of my drive is WDC WD2500JS-40TGB0.
There's definatly no high pitch sound coming it, but according to my tests it is still probably the loudest component in the imac. I found an app that allows you to change the fan speed, and tried changing all 3. I did not notice a big change in the sound level with small increases to fan speed, so the only thing left is the disk.
But personally I don't find this to really be louder then my notebooks, maby it's just that the sound has a lower steady frequency. By notebooks I mean a last gen G4 powerbook, macbook pro, HP NC 8000 and a fujitsu business class Centrino. All pretty silent. Maby the powerbook was more silent...
Silence is relative, of course! As I sit here, with the house heating activated and water gurgling through the pipes, I can't really hear the iMac, aside from the faint rattle of disk access and a slight ticking noise of one of the fans.
Normally I work in a completely silent office. You've heard the phrase, "You can hear a pin drop"? That's my office during working hours. I even took a clock off the wall because it was making too much noise.
In that kind of silence, yeah, I can hear the iMac. In that environment, it's far from silent.
When using my iBook G4 in the same room, I can just about hear its hard disk, but it's a million times quieter than the iMac. I've never used any other computer in the office. I would get-out my old Compaq notebook for testing but I really can't be bothered.
Maybe I ended-up with a dud iMac, I don't know, but the final word from me: If you want a quiet computer, get an iMac. But don't expect it to be silent. I even think saying, "virtually silent", as seems to be constantly said about the iMac, is pushing it. I would say the iMac is quiet compared to most off-the-shelf computers.