Originally Posted by
prairiepedaler
I'd say it's some part of the frame flexing. Happens to me too when I wind it up.
If by "wind it up" you mean spin at a high cadence, I would have to say I find the opposite to be true. I notice the flex the most under high torque situations, at lower cadence right after shifting before a sprint or when climbing. I had an el cheapo square taper crankset on my old single speed and I actually managed to bend the ring beyond repair from putting so much torque into it.