I have the exact same model as you and there is always a humming coming from my iMac, I can certainly tell when it's on if I'm in the room.. It's not really loud but it's noticeable and my fans are running normal speed. Sometimes it will pick up and be a bit louder and sometimes when I go into folders I haven't been in for ages it will make a right racket and take a few seconds to show up all the files.
I can't be bothered taking it in for service because:
1. It's too big
2. My nearest Apple Store is about 70 miles away.
3. My Apple Store is in the middle of a shopping centre, so it's quite a walk with it too..
4. I'm only 14 and it's hard convincing my parents that this small noise is a problem.
Hmm, but you don't have to take it to the Apple Store yourself. Here in Europe you can make a call and the guys are going to pick the machine up at your home. In fact, you don't have to do anything, only to describe your problem on the phone. At least, that's what I've been told....