There are 4 revisions of 24" iMac with various configurations. Which one is yours?
I can report the 2008 MB325 2.8GhzDuo has a slight buzz going on after a year and a half of use, my 2009 MB420 3.06 has a very little noise at all, lovely thank you very much.
I reckon if it is actually 'noisy' then get it in to the Applestore if possible. I've had various iMac configs and the first 1.8Ghz rev a was quite famously a real blower!! In fact in summer it was not possible to watch a dvd on it, the noise it made sucked big time. Luckily for me the capacitors on the mother board blew out after the 3 year guarantee was up and I had to trash it, forcing me to get a new machine.

Oh, the Apple dingbats said the noise was acceptable on it when I took it in even though I reported that the serious disturbing noise comes after an hour of use etc. Hope you get a better result since years have passed eince then and Apple have a bit more money to help customers.