It's also possible that something is just loose enough in it to vibrate from the 60 Hz vibration/hum of the transformer (if you have one with a large transformer in it). Some of the newer models that are sine wave do not have the large transformer. At any rate, I'd keep an eye on it. As Gary said, it could be going bad.
I've got the same thing going on in my Sharp microwave/convection when the magnatron kicks on. It buzzes like a banshee for a couple of minutes. Then, what ever it is, heats up/expands enough for the buzz to stop.