Something loose in the engine.

[eric81]

It is most likely the motor mounts. If you look at how they are made and put together, they move front to back AND up and down. It doesn't take much to jiggle the engine up and down. I am betting that your mounts aren't on quite right, or they are bad from the factory. It's the only thing I can think of. That and the battery, but that would be obvious... just grab the battery, and push really hard towards the back, then yank on it to bring it foreward. If it moves, tighten down the 10mm nut on it.