* David, the valve clearance is adjusted hydraulically by "hydraulic elements". There's an initial setup adjustment (via thrust shims) but no adjustment after that.
* One hydraulic element is located in each rocker arm (M103), 12 hydraulic elements in total.
* The tapping noise you hear might be a hydraulic element which won't "pump up" but rather drains off it's oil too fast. The elements can be replaced without removing the cyl head, but you will have to remove the rocker arms.
* Get yourself the MB W124 dvd or the factory M103 book, if you can find an old one around, and read through the replacement process in detail. You can do some damage here if you mess up cam timing or cam bearing torque. Also keep track of which thrust shim goes with which rocker arm. Good luck.