My (new to me) '68 Dart got loud between 45 - 70 mph and you could feel a rolling vibration through the seat and steering wheel. Felt like driving a log truck! I swapped rearends, checked the driveshaft, tried different wheels/tires and no improvement. Yesterday a couple of friends finally traced the cause - my car had the wrong transmission cross-member so there was a steel lip making direct contact with the transmission case - about a solid inch of interface. We modifed the cross-member to make clearance, and - bingo - it's now a quiet car. Even at start and low speed there is less noise in the car. I was almost ready to swap another rear-end, so I'm very relieved to find an easy solution! Thought I'd pass it on, may save someone else some time and money?
Bill

Gees Bill, great ending to your story - thanks for sharing

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Gary from Downunder
2 weeks ago I couldn't even spell mukanic - now I are one!