To the OP: My MP (2.8GHz Octo with the 8800GT graphics card, stock Seagate 320GB HD, 3 x 750 Samsung Spinpoint F1s, 2GB original Apple RAM, 8GB OWC RAM) also make that (low) humming fridge sound. It seems to come from the back of the case.
Actually I've just been to a Mac reseller. I checked out one of their MPs on display with the stock ATI card (which should be less noisy than the 8800GT). I could hear the front fan quite easily, and I could also hear a humming sound. And that machine was idle. Thus I'm certain that there's nothing wrong with my MP.
However my 8800GT graphics card makes a lot more noise, and that is my concern (I'll look into ways of making it less audible). Also my HDs cause a lot of resonance, which should be easily solved. I'm not sure if it matters, but my MP was assembled in Ireland (I read elsewhere on this forum that someone had bad experience with the MPs assembled in Ireland).
I tossed away the original shipping box for my MP shortly after receiving it from the Apple online store, and I don't have any intentions anyway to return my MP (it works great - temps are always very low and the fans are happily spinning at their default settings). I need my MP for daily work, so returning it and waiting for a replacement is not an option anyway. Takes too much time.