Auto shifting hard & thumping

My 3 speed auto seems to shift hard if I snap off the throttle from heavy acceleration, particularly if I'm near a shift point when I let off. It's like a light kick in the butt and there's an audible thud.

It only does this when I let off from heavy throttle - it operates normally otherwise, even shifting at WOT. So... what might it be? Torque converter? Loose mounts? I've already checked for the obvious body in the trunk (not that there's much room for one anyway). Thanks.