musty smell from new ac vent

I'm building out a bonus room right now and had a few new ac ducts installed. One of those goes into the new bathroom. Since they were jumping new ducts off of the existing ac, it was a tight fit to get this one in for the bathroom. They wrapped the duct in some sheet metal and then sort of forced it thru a very small wall/ceiling space on an adjacent wall to get it where it needed to be.

Ever since I had sheetrock installed (now I'm painting) the air coming out of that new vent smells a little dank or musty. I stuck my hand inside the vent as far as I could to see if I felt any water build up or anything, but there wasn't any. I've got a call into the ac company to check it out, but in the meantime I'm obviously thinking what could it be.

Thoughts?