Malfunction of the non-return valve leading to stoppage of M/E

Summary:
Banging / knocking sound from MIE Unit when RPM is reduced to half level. All normal at higher RPM.

Corrective Action/ Lessons to be learnt:
Checked the fuel injector after stepping into the engine level normal, then checked the exhaust conditions. Exhaust valve was opened and Found lot of oil accumulation inside the upper air chamber, observed malfunction of non return valve and renewed the same.
Follow the OEM instructions regarding exhaust valve o'haul and the parts to be replaced as per running hours.

Incident:
Had to stop the M/E for 4hours to rectify the problem.