Is it the engine backfiring through the exhaust or is it a mechanical
clunking?  Your description does not make this clear.

 Bob

Sounds like a bad or incompletely installed (engine or motor was
recently pulled) motor/tranny mount.   If that's not it, check
suspension parts.  But since you had recent work done that would involve
the mounts, that's where the suspicion is.  If you open the hood and
watch the engine while someone guns it in gear with foot firmly on the
brake, you should see the engine move a bit - that would be what you're
hearing and feeling.

Take it back to the shop that did the work and let them handle it if you
feel it was a mistake and not general incompetence.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
adddress with the letter 'x')