Hello out there!

I have a 1990 420 SEL and it is making a tapping noise like a stuck lifter. I had a mechanic take the valve cover off and he said it wasn't anything in the valve train. All lifters were working, and there were no broken rocker arms. I don't think he really knew Mercedes. So I took it to another certified Mercedes Mechanic and just listening to the car he said it might be a shim problem or a valve may be out of adjustment. I'm taking it back to have it investigated further next week. It's a very solid knocking sound but definately not rod knock. Does this sound familiar to anyone?

thanks