Tappet noise is a ticking, tapping, clicking or clacking noise that occurs when tappets, also called lifters, hit the top of the engine, and this can cause engine misfires and damage. It is usually heard in the engine half on the passenger's side, or the right side of the car in American cars.
Tappet noise can be loud and irritating but can also cause problems with the performance of the engine, damaging the camshafts. It is often caused by missed oil and filter changes or the incorrect grade of oil in the engine. The cure depends on how much damage has been done, but an oil and filter change that uses the correct oil for the engine is the first thing to do.