Back in 69, I was working at the local Dodge dealership and a new car was purchased by an elderly man. He drove it about 100 miles and brought it back in for a "shake". I balanced the tires, took it out for a drive, and brought it back in still with a shake. I took the driveshaft off and took it to be balanced. Got it back, reinstalled it, took it out for a little drive, it was fine. Guy comes back a week later with the same shake. I opened the driveshaft, it was filled with newspapers, and a D cell battery.
WR