Tapping noises coming from an engine may be an indication of sticky hydraulic valve lifters. Engine valve-train components include the camshaft, hydraulic lifters, rocker arm assemblies, valves and valve springs. Overhead valve engines also have push rods that ride on top of the lifters. Defects in any one of these components may cause tapping noises, but sticking hydraulic lifters will cause definite tapping or knocking noise and will affect engine performance.
Hydraulic valve lifters ride on the camshaft lobes and transfer the rotating motion of the camshaft to the push rods. Push rods move the rocker arms up, opening each valve in the correct order. Unlike solid lifters, hydraulic lifters are able to maintain zero lash in the valve-train, providing for quieter engine operation and smoother engine performance. Each hydraulic lifter has a metering orifice and a check valve that controls oil flow through the lifter. Changing oil levels in each lifter provides a means of adjusting to variations in engine operation.