Just FYI -
One of the quietest narrow 50-pin SCSI drives that I've ever tested (and I've tested > 50 different models in the past) was Seagate ST318418N - 18gig, 7200 rpm. The other one was ST318417N, also quiet, just slightly slower. Both models are (were) relatively recent - I think I've tested them in 2007, on loan from the dealer. I kept the ST318418N and now it resides inside the overclocked Q605 @40 MHz. In fact, Q605's fan is louder than the drive.
So, Anders, just google a bit and I'm sure You'll be able to find one somewhere. At the end it's much more elegant solution than all that IDE/ATA/adapters etc. hassle.
Just make sure the boot partition is 2 gig or less and away you go
ojfd
P.S. That Q605 mentioned above is a real workhorse - it runs Cubase 3.0.1 (not XT and not VST, just plain MIDI) with two Opcode's Studio 4's under MROS (no OMS) and it's timing is almost on par with ST1040