What was found
UPON DESCENT, CREW NOTICED UNUSUAL NOISES COMING FROM THE ENGINE COMPARTMENT. ENTERED A POWER-ON AUTO ROTATIONAL PROFILE AND LANDED IN A NEARBY FIELD. MAINTENANCE REPORTS, ENGINE HAS A NOISE AT FULL THROTTLE, FLAT PITCH, THAT WILL INCREASE IN INTENSITY WHEN TORQUE IS APPLIED AND LOUDEST JUST AFT OF THE EXHAUST COLLECTOR. THE NOISE WAS DESCRIBED AS A HIGH PITCH GROWL TYPE SOUND THAT OSCILLATES. THE CREW IN THE AFT CABIN HAD FIRST THOUGHT THE WHELAN SIREN WAS ON. REPLACED ENGINE.