the two of the most quietest drives I know are:
Samsung F1 (also quite cheap) uses less platters thus less seek time = less noise

IMHO
Western Digital GP drive brilliant drive and the most compatible drive) I have two of these myself and one in my tvix player and i can't hear it at all)
how this works is it adjust speed automatically so takes less power = less noise and better cooling. runs between 5400-7200rpm more than enough to run media even at 5400 rpm.
I bought the Samsung f1 first had compatibility problems with my motherboard (nvidia chipset) so returned and bought WD GP best drive.
Seagate are quite loud drives but for performance very good.