As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) ON ENGINE SHUTDOWN, A SINGLE POP SOUND IS HEARD AT THE EXHAUST AREA. FURTHER INVESTIGATION DETERMINE THE DRAIN / OVERSPEED VALVE TO BE THE CAUSE. A SMALL JET BENEATH THE T-FITTING WAS FOUND TO BE MISSING. THE DRAIN/ OVERSPEED VALVE HAD JUST BEEN REPLACED DUE TO 10 YEAR CALENDAR LIFE