I get he same sound now and then. I seemed to come from the trunk area as apposed to under the rear of the car. On investigation I found the right trunk lid spring would shift a little each time the trunk was opened.
Then I slight bump or sway would let the spring correct it's position with a
"sproing' The easy fix, a little grease under the spring hooks

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1984 300SD turbo 126
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God made me an atheist and who am I to question His wisdom