The first iMac I purchased had the same problem. I sent it back because in addition to the buzzing, the CPU fan never dropped below 2000 rpm and it had lint behind the screen in three places.
The replacement that I received has the buzzing too (not as bad as the first), but it has to go back too because the stand is so loose the screen falls to the bottom every few hours (I know it sounds like it should be so simple to just tighten the stand, but apparently its not because apple told me they would rather just replace it).
Anyway, hopefully third time is the charm, but both of mine have buzzed, so Im not too hopeful.