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iTrader: (4)

I do not know if this is the correct section to post in but here it goes. I just had my exhaust changed over from a true dual back to a 3" y-pipe to a 4" intermediate pipe over the axle into the muffler with a single 4" tip on the drivers side. Here is the problem when I am in OD and only between 60-70mph it sounds like a sub thumping whomp whomp whomp. This noise is just starting to be audible at 56-57mph. Now if I leave the car in D I can run right up to that mph and have nothing. Now in D my rpm's are between 3500-4000 and in OD it is 2200-2600. Is there some kind of harmonics that is just happening here in OD. The 4" pipe going over the axle is really close and is wraped with header wrap to keep the rubbing down to a minimum. Is there anything I can do to try to eliminate this problem? The exhaust hanger that is normally right by that bend was not used. Would welding a tab and connecting that hanger help out?