PROBLEM SOLVED!!! Looked in the tank and found that the vent tube was intact. Not very stable, but intact still. Took the tank off, and discovered that the rubber tank bushings had rubbed themselves out of round, and worn themselves down completely on one side, and had a terrible vibration and rattle. Took some emory tape and wrapped it tight until everything was fitting tight and re secured the tank....rattle and vibration gone. Will order new bushings, but also discovered that the tank sits directly on the frame, so we'll have to put a strip of 1/8th inch strip of rubber on the frame between the tank and frame.