If it is coming from the engine and you have the old 4 cam 32v
M119 V-8, there is a pretty good possibility one of the valve tappet oil delivery-tubes has lost it's end plug.
The oil to one lifter is lost, oil pressure drops and the lifter starts
"tappin" . removing the valve cover and inspecting all the little
brown plastic tubes between the cams is a good place to start.
if one is missing the little plug at the bottom, that is your culprit.
If one has failed, replace them all. pn# 119 180 03 66.
AGAIN, THIS ONLY APPLIES TO THE OLD M119 32V V-8.
and is a mere suggestion of a known problem.