KNOCKING
Engine knocking can indicate a very serious problem in your engine and should not be ignored. Some causes of engine knocking can include compression issues, faulty fuel injectors, timing issues, failed bearings, or failed wrist pin bushing or wrist pin.
COMPRESSION ISSUES
A lack of compression is one of the biggest indicators of an issue within your engine that needs to be addressed. Compression issues can reveal leaking/broken valves, leaking/broken piston rings, blown head gasket, camshaft issues, broken timing belt, or a hole in the piston or cylinder.
SMOKING
Excessive smoke coming from your exhaust can also clue you in to a larger issue in your engine, especially blue or black smoke. This can indicate quite a few problems, including faulty injectors, faulty injector pump, bad air filter, bad turbo, carbon build-up, poor or incomplete combustion, worn valve guides or seals, or wear in power assemblies.