Top engine mount?

Engine mounts (including the one at the tail of the tranny) are my first
thought also. They should be firm and nicely cylindrical, deforming slightly
from the weight. With age they will smoosh (industry technical term)
sideways. The normal failure mode (the one you would be looking for) is for
one of the end plates to rip off from the rubber biscuit - that is visible
as the two not exactly lining up any more. The offset may not be great, so a
careful look is sometimes required. The best indication is the same as the
setup for replacing either front mount: place a 2X4 or wood block between a
floor jack and the oil pan and lift the engine a bit. Broken mounts are
pretty obvious then.

The front mounts can be frustrating to change, especially the right one, but
the rear mount is easy. Since the thump is when you hit the gas, I'd focus
on the left mount.

Mike