Rim true and or derformation of the tire. Maybe only one.
the Rev's got more expierance..probably right.

My idea is the deformation causes a small oscilation\ wave in the frame, this travells the length of the frame and returns, runs length again and is eventually dissapated unless........the speed of the knocks is the same 'bpm' as the wave travelling.
then you don't have a wave with more (closer) peaks, the energy is being added TO the wave peak.

Like a long wire stretched, a hit will cause a wave to travel up and back, then up etc.
if I were to add more energy to the wave as it passes, i'll cause a larger wave, if I hit it not during the pass, i just make more waves but the intersity does not increase.

A buzz could be a big wave if the peaks align. This happens when the speed of the wheel vibration is at the same speed as the ocilation of the wave traveling the frame.

Whew..well.....I did give it some thought.