The theory about the supersonic compressor fan blades is correct. At higher RPM, the tips of the blades are breaking the sound barrier causing the buzz. This is the same thing that makes helicopters and prop planes so loud. I've always noticed it most on the airbuses, but have heard it on 57's too.

Had a full power takeoff from EWR in a 319 last week while sitting in 1D. This is probably the exact sound that earned these planes the nickname 'ScareBus' among the flight crew. Kinda reminds me of the constant prop drone in the movie 'Airplane'.

The most unsettling sound is on the 320 when the hydrolics go thru the self-test. If you're over-wing, it sounds like there's someone in the cargo hold with a sledge hammer trying to get out. Sometimes the pilot will warn the passengers, other times it's fun to look at faces when you hear the sound!