1990 Ram 50 2.4 4cyl RWD automatic transmission noise

When my 1990 Ram 50 is started up 1st thing in the morning I get a loud chattering sound for 1 or 2 seconds when I put it in reverse to back out of the garage. Then the noise stops and the truck runs fine and no more chattering when putting it in reverse all day long. So, only happens once when engaging reverse 1st thing in the morning. Could it be something in the transmission leaking down over night? There are no external leaks on the transmission or the engine. The truck only has 19,000 miles on it from new. (it led a very sheltered life)