[jimcol]

I had a similar issue once with the sweating ceiling. Luckily it wasn't very far from a ceiling diffuser. Popped the diffuser out and could look in and see the duct had separated or at least was letting hot attic air into the cold duct. Appeared the tape on the outside of the duct had come off. Was able to contort my arm enough to get in there with some Aluminum tape and seal it back up. Solved the problem.
Perhaps a snake scope like member Daryles often recommends will allow you to see further into the duct.