When I start the engine it runs smooth, but some times I heard noise like tappet sound in idling, but if I raise the rpm, then it become smooth running, Then sound like injection & tappets, How can I clean the lubricating system with out open an engine. please, advice me..I changed water pump, coolant belt, new battery, belt tensioner,, flushed oil system with flushing oil, and replaced new high quality oil with filter, previous owner not maintained well, Oil removed from engine was black stick, My car is Chevrolet Lumina model 2006[commodore]