Just a single clunk - not normal

2010 Prius with about 145,000 miles on it. It's recently started making a noise and I would like to narrow it down before taking it to our local mechanics. I hear a sharp clunk noise when accelerating after a stop or near-stop. It doesn't sound metallic, it sounds like a pebble hitting a piece of plywood. It seems to come from underneath the car, near the center/driver's seat. I can actually feel it in the back of my seat sometimes.

Weird thing is it doesn't do this until the car is warmed up a bit. Yesterday I drove about 15 miles round-trip on an errand. First five miles or so, lots of stopping and slowing for sharp turns, but no noise. Once it started making the noise, it made it every time I moved from stopped. The seat gets moved up and back regularly depending on who is driving, and I didn't see anything obvious under the car. Someone mentioned it could be the ICE coming on, and I will watch the graphic display next time I'm not driving to see if that's it ... oh, I could put it in EVO mode too, I guess, once it's doing it reliably. Any other hints about what to test/look for?

 

A rock, that's a good idea ... we live on a gravel road. Will check that! Is there a way to check the CV joint myself?

 

I took the car into a local mechanic. He said he heard the noise while the car was on the lift, and gave me a copy of a TSB: T-SB-01560-14 which just says "HV Transaxle Repair Components Available" and suggested it might be a dealer silent recall issue. There's no explanation on the TSB, just the list of parts. Does anyone know more about this TSB?