Hum on landing

On my last two flights I heard a hum develop after crossing the threshold on landing. Not transmitted through controls, Not violent. Disappeared on touchdown. I guess the frequency was 100-200 Hz. Plane is a 415C C85 and speeds on landing were correct. Engine rpm about 1,000. On ground inspection of play in flight controls seem to be within spec and hinges seem OK. I am guessing it is a result of landing with windows open (pipe organ effect), side window vibration, or vibration of one of the wing inspection plates. Will try to sort out the window ideas on next flight. Any other ideas?

Ted Gordon
N 99164