I did the same upgrade last week, on a 400 MHz iMacDV (slot-loading). I installed a 7200rpm 80 gig, 8meg cache Seagate 7200.7 drive. It works fine, and is about twice as fast as the original drive, and there is no annoying high-pitched whine. But after 45 seconds or so of inactivity, it grinds very loudly for about two minutes before stopping. Then it repeats forever, until the OS puts the iMac to sleep. Causing disk activity (just clicking on stuff) also stops it right away.
There's no OS activity (indexing etc) going on during this, the machine is really idle. The same thing happens sitting at the 'choose a drive to boot' screen via option-boot.
I've read a few things from 3rd parties so far:
* this is normal behavior designed to seek out bad media blocks during the first ### hours of drive usage. It should stop after a while. (I don't know if this is true, or what ### is, though.)
* there is a DOS utility called SMSCAN.EXE to disable this media check. (I don't know if it works, since it requires installing the drive in a PC.)