Um have you tried it with the brakes slightly applied and all the way off?
I had a similar problem and it turned out to be a worn Brake pad that came slightly loose and was rattling. It could be loose Suspension parts like a totally worn bushing causing metal on metal contact, it could be a shot whell bearing but you would hear other sounds liek roaring sounds when driving. the Lugs could be worn causing some weird vibrations but I doubt that because again at speed you would feel it. And your sure it's not the exhaust when the rear is under compression?
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