Just got my replacement iMac i7 with the 2GB Hitachi HDD. Same story - loud, low-frequency, constant noise from the hard drive.
Fast processor, beautiful screen, excellent speakers (actually, the speakers are better on this replacement one, I'd describe them as "amazing" for an all-in-one). But - I don't think it's possible to work on it. The constant noise drives me nuts.
Thinking about returning for a refund. Will be genuinely sad to stop buying these beautiful Macs, but 2 faulty ones in a row...
hmmm your the first with a loud hitachi i find that really surprising, especially that i have just ordered one to get away from the noise