Problem with Main Engine Exhaust Valve Operating System

Ship came out of drydock (routine drydocking). On coming out of the dry dock, the main engine was started and found working well. After few minutes, Unit 5 exhaust valve was found knocking, and the sound was increasing. Suspected air lock, the hydraulic system of purged off air, but found no improvement. On investigation it was found that no oil was coming to the piston. The lines dismantled and thoroughly cleaned. Waste pieces were found to block the passage.