Brand New 27" iMac: humming noise!

Hmm, very strange. My previous pc had a similar problem a few years ago, and that turned out to be related to the fact that power cord was nog earthened/grounded (ho do you say that in english??) - it should be. It might also have something to do with some kind of interference - have you tried to put your Imac in a different place/room?

As strange as it sounds I believe you just got another bad one, this is very unusual. I'd recommend taking it back for another exchange however this time try out the new one before taking it home. I'm sure the store will be happy to help you with that.

BTW I was in Apple stores in the past and think it would be very difficult
to hear and demonstrate the noise in such ambient noise areas.

They could take it in the back to listen, somewhere where there wasn't as much ambient noise.

I think I'll call Apple tomorrow and ask them how to get a quiet one.

You're welcome you to call of course however I doubt that call will be very productive.

coxorange:

It seems the buzzing and humming problem computers are in Europe and not the U.S. yet. Were they made at the same factory and where? I guess we'll have to know which week(s) of production?

well, I'm in Europe (The Netherlands) and the Imac that I received last week doesn't have any problems (so far, fingers crossed...). I couldn't find any information on the box about the production week. Anyhow, it may just be a case of bad luck. Hope Apple can help you!

When I set up my new 27” iMac on a heavy dining room table, the hum was inaudible. After installing more RAM, I moved it to my computer desk and the hum was annoying, (I had no hum from my previous 24“ iMac on the same desk.) Setting the iMac stand on a mouse pad to decouple the sound didn't help, but the hum became barely audible after setting the iMac on a 1 x 6 x 12 cedar board. The desk had functioned as a sounding board.

I have exactly the same problem (and exact the same circumstances). In fact I have to pull the Mac towards me because otherwise it is located under a bookshelf (about 20 cm above it) and because of the acoustic properties of that specific spot the hum becomes unbearable. I do not experience any vibration though (when I lay my hand upon it).
I do suspect the power transformer too; it does sounds typically like that. I'm gonna call Apple about it.
And no: this is certainly not normal! (and I wonder why you hope that, but that is something for another forum, lol).
Furthermore at startup the white screen (before the actual start-up process begins) stays up for uncomfortably long period, but I guess that has little to do with the hum-prob.