Hello Fulcrum

I can't answer the mechanical side, but you are not alone. My previous sedan (4-speed) became really boomy at about 55-60mph and this disappeared at about 75mph, being replaced with just higher engine noise - consequently I either had to dawdle on the motorway or speed. Now my 4-speed with overdrive booms at about 70mph in overdrive top. I figure it is the engine revs finding a reverb with the body, which depends which gear you're in. Do you have carpet, or some sound-proofing in the firewall and floor area? This might help. Whatever people say I've yet to drive an Amazon that is 'talking/classical music quiet' at 65-80 mph, because if the engine-noise doesn't get you, the wind-noise will. But then again you don't buy an Amazon to appreciate Chopin in.

What I would like to know is, can you do anything else to windows and doors to try to make them completely air-tight, apart from just replacing the rubber surrounds?