When I'm cruising at 45 mph or above, and begin to lightly and slowly accelerate, my old '91 with 200,000 miles, begins making a noise I can best describe as sounding (and feeling) like someone is working on the under carriage with an air hammer. The noise continues until it auto downshifts. Before the noise begins, if I accelerate hard enough to force the automatic trany to downshift, the noise will never happen. After the noise begins, if I accelerate hard enough to force the auto-trany to downshift, the noise will stop.
This pattern occurs when I'm in OD or D and seems to happen more easily once the car has warmed up.
There is no lose of power during the noise.
Trany fluid level is correct and the fluid looks clean.
Does anyone have any idea what this might be????
Thanks,
Mike