My 2232BW is exactly the same - an electrical hum that increases in volume the lower the brightness gets. At 50% or so it becomes inaudible. Apparently this affects quite a few panels, not just Samsungs, and it seems pretty random.
I decided not to RMA mine, though. I've set mine to the 50% brightness which I find fine for both day and night-time usage - and I figured that since the panel on mine is flawless, I'd rather not take the chance of getting a replacement with a dead/stuck pixel etc. Since there's a 3 year warranty on this, I can always return it at a later date if the hum gets worse/causes other problems.