Engine noise under the hood may be caused by loose torque converter bolts, low oil pressure, spark knock or excessive piston clearance. A cracked flywheel on an automatic transmission or excessive connecting rod clearance may also cause an engine to knock.
When an engine starts knocking, one of the first things a mechanic will check is the oil pressure. A warning light may come on in the car, and adding sufficient oil will solve the problem.
Sometimes, a knocking noise is caused by loose torque converter bolts or a loose flywheel. Loose bolts can simply be tightened to get rid of the problem but a faulty flywheel will need to be replaced.
Spark knock can be caused by using a fuel of lower quality or by carbon buildup, an ignition timing that is set too high, or a bad EGR valve. Each of these will need to be checked to determine the cause.
A feeler gauge can be used to determine the connecting rod clearance. Excessive piston clearance is usually caused by worn cylinders or worn pistons.
Some piston knocking when the engine is first started is normal but any knocking coming from the engine after it's been running for a minute or two is usually sign of trouble and should be checked out.