Get back under there...
Just re-do the connections to the H-pipe. Undo both of them and make sure that they line up correctly. Then tighten them down a little at a time. Switch sides back and forth until you are sure they are tight. For insurance you can add 3-4 washers on each side to get them to tighten earlier. I have screwed with this problem on a friend's car more than I would like to admit.
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You're gonna need your seatbelt
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