If you're using a 2.5" drive then you'll be in a desktop case which has vents in the top. This lets through the high frequency noise of the drive and is pretty unbearable.
I've tried Quantum, Hitachi and IBM and they're all very whiney.
(Note: Quantum Daytona Go-Drive is not Amiga compatible).
Seagate seem to be the quietest drives but are also the hottest. I sold my Seagate Barracuda 3.5" because I was worried about the severe heat it gave off.
Trouble with using sound absorbing material is that it'll block the natural airflow and might overheat your Amiga.
The best route nowadays is to get a Simpletech or Silicontech 2.5" ATA flash disk. This looks very much like a 2.5" laptop drive and has the screw holes in the side for fitting into an A1200/A600 cradle. However there is no motor inside, just a bunch of chips.
I've seen them in 448MB, 540MB, 800MB, 1GB and more sizes. I also saw a site with SCSI-1 versions.
They're half the height, noiseless, heatless, long lasting, impact resistant and have ultra-fast seek times.