I would agree with JRD10, sounds like it is vibration in the Z Screw. I thought i might have a similar problem but i have been able to come up with a solution. My ripples are diagonal, not horizontal on the Z axis. So i believe it is a problem with my extruder under extruding then overextruding. I have adjusted and readjusted the extrusing in firmware and tried messing with the estusion multiplier with no success.
Anybody got any thoughts?
So did this actually solve your problem? 
