Could be just the exhaust hitting the body, which can make a very noticeable sound. My 1984 does that the few times I had two big guys in the back, compressing the springs. Happens mainly at idle. I tried to reproduce it, having my wife stand on the rear bumper as I looked, but couldn't find anything in the exhaust that could touch, so perhaps a worn rubber bushing that allows metal-metal contact under load. The independent rears in these cars is more complicated than the simple solid axle - leaf springs of most 70's U.S. cars.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans