I'm selling quite a lot of HPs, ACERs and ASUS, and I never saw one of this manufacturer swap or repair under warranty a computer just because the hard disk is "noisy". Never.
They repair it only if it is DEFECTIVE.
I think a lot of people is abusing of Apple Customer Care, in this specific issue (the REAL yellow tinge is another story).
I know I'm going to hurt someone feeling in saying that, but this is only my opinion based on my experience in this field
I had a 24" replaced with a 27" because it died on me. That was back in November. I've actually thought at times that I wouldn't mind just getting another 24".
I still don't have a working 27". And as for the HDD noise, it is a huge difference from the C2D to the new 27's with Seagate HDD. I never heard my 24". Some may have gotten quiet machines, but I can assure you that if you had one that was loud, you'd know it. I had to upgrade to a 2TB drive to get the quieter drive. Unacceptable.
i have the same model, 2.8GHz Penryn. it has a 750GB HDD.
this model of iMac is always quite loud, its pretty hot here in australia. ambient temps of around 30°C+ somedays. its certainly much louder at idle (when fans are up) then when my imac i7 is under load!
thats just my personal experience/opinion though, of course.
You have the Core 2 Extreme Version, I have the 2.4 Ghz Regular Core 2 Duo. So yours probably generates more heat, which cranks up your fans. And Since your HDD is of a different capacity, we probably have different (and you have the louder one)
I have a 1GB Seagate in my iMac 27" i7. It's makes that stomach rumbling sound but I can't imagine why that would bother anyone; it's perfectly normal.