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And yea, the return will pop if they didn't break the duct. If it's popping you can fix it by getting some cheap small gauge angle iron and screwing the angle in the middle of the duct from one side...

For one, they screwed you because it isn't even insulated. They screwed you buddy, you always insulate duct if AC is on the system whether it's in a basement or attic, no matter finished or...