I work at night in absolute silence - so a quiet compter is important to me. I was aiming for the benchmark set by my 24" alu iMac - a very feint rumble on write/seek.
I've got a 27" i5, with a 1 TB WD 'Black'. A fair bit of seek/write/read noise (more noticeable than loud, though), silent when idle. I thought about complaining, but wasn't sure I had a complaint.
I've tried a WD 1TB 'Green' - better, but still obvious write/seek/read 'grumbles'. I tried to source one of the Seagate drives, but Seagate referred me to Apple, Apple don't sell HDs. Now, I'm using the WD 320GB out of the 24" iMac. I've put the WD Black in an external caddy for the (rare) occasions I need the space.
No idea what to suggest. I think it's a combination of things - people's reaction to noise and the environment they have; batch variability in HDs; and noise of the HD in any case.
And: 1) iMacs are normally class-leading quiet. If it wasn't for the familiarity of my 24" iMac, this wouldn't bother me, and 2) Opening up the iMac and replacing the disk is a scary business! It's easy to damage the screen/connectors, and in the case of the 24" I've had to replace some tape, so it'd be obvious I've 'been in', so the warranty may be affected.