How many miles on it? The tapping could be a lifter or rocker arm or camshaft. Did you pull a valve cover? The cam can wear on the back cylinders so that the cam looks good through the oil filler hole, but is worn out. I am not sure but I think the left rear ones are the furthest from the oil pump so they get oil least and last. This is done by plastic tubes and theycan clog as well. The lifters can be shimmed and I was told they almost never go bad. Most unlikely a rocker arm. My unskilled 1.5 cents.
'85 500SL (Euro)