That's a tough one. Sounds definitely like a resonance problem and I would suspect propshaft mounting, but you already changed that. Do you feel resonance in the cabin, or is it sound only? Sound maybe a standing wave in the exhaust. Try going uphill and downhill in 5th and 3000 rpm and see whether that makes a difference (you should have different throttle positions then). Otherwise, the loading of 5th gear at 3000 rpm must be causing some kind of engine and/or transmission resonance that gets transmitted. Have you checked all clearances around engine and transmission?
Good luck,
Bert
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'70 111 280SE/c 3.5 (4 spd manual) - sold
'63 MGB
'73 MGBGT V8
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