An experimental approach was undertaken to investigate a low speed “diesel sounding” knock of an L-4, 1.6 litre, gasoline engine. Specific tests were devised to show the noise source to be associated with piston and ring friction. It was concluded that the noise was caused by a “stick-slip” action of the piston assembly as the piston reversed direction at the top and bottom of its’ stroke. The instantaneous change between static and dynamic coefficients of friction resulted in an abrupt force reaction, which excited the crankshaft’s natural frequencies. Design parameters were optimized to provide a 10 dB(A) (A-weighted sound level) reduction of noise at idle.