Acquired my 2nd van recently ... now I am a bona fide enthusiast. It really is a gem for the price i gave. It's biggest issue is that at 50 mph it gets loud with vibrations seemingly coming from the rear end. Mysteriously, it smoothes out at 60. Runs great at speeds below 50.

I trailered it home from out of my area for this reason and haven't tested it on the open road . Any thoughts or diagnosis?