I'm on my 3rd iMac. The 1st and 2nd were taken back to the store for the display dim buzz that has been well documented around these parts. The new machine seems to have the opposite issue that the first two did (in addition to some others). When the display is dimmed, I do not hear an electrical noise. However, if I increase the brightness, I hear a high pitched whistle...the whistle is not sustained, or perhaps it is and I have difficulty hearing it over the relatively loud hum (and distinct whirring) of the fan(s). To me, this is also a problem, as I could not hear any fan noise with the first two machines at all. All of the machines had some normal sounding HD clicks, only the one I currently possess will grumble for sustained periods. All of these sounds combined are really starting to drive me crazy, and I'd really just like to have one of the machines I had exchanged back. I don't think I should have to feel that way.
In Apple's defense, the customer service at the retail store was excellent and the geniuses were very helpful. They made the exchanges as painless as possible. Also, none of the 3 27" iMacs I came home with had any dead pixels, gray bar, or yellow tinge issues (that I noticed).