Engine noise

With no other details to go on the logical guess would be a spun connecting rod bearing. By momentarily grounding a spark plug wire you may be able to figure out which cylinder has the problem. If it's a double-knock each crankshaft revolution and it goes away when the cylinder is disabled, suspect the rod bearing. If it's a single clunk that doesn't go away, (on a V-type engine), it's likely a main bearing.