What additive can I add to my oil to quiet tapping valve?

 

Tapping valves would be unusual. It is more likely to hear a worn VVT. You could use a higher viscosity oil, but if you are convinced the valves are making the noise you should check the clearances and adjust.

 

Not having hydraulic lifters it is, indeed, rare to hear of tapping from the valve train. As RJ237 says, it is much more likely that the sound you are hearing is from the VVT's. The standard answer to your question is to add a can of RESTORE to the oil. If that quietens the noise then move to a higher weight oil at the next change i.e.: 5W40.