Unfortunately it's perfectly natural for a shop to screw up and not own up to a mistake. Ask them again; bring donuts this time...
I have no idea. It could be that, or something as simple as a tool left on top of a heat shield. But according to your OP, the noise was not there before the shop did the work, therefore they are best equipped to find and fix the problem.Thank you Yadkin for the reply, Do you think bolts or something could be loose? I am going to talk to the owner next and see what he will do.
Good guess. The exhaust has to be removed to do tranny work...Exhaust pipes rattling?