A PA28-181 instructor reported a violent; very loud slapping noise during descent with the engine in...

Narrative:

After nearly 2 hours of flight during the descent to my home airport the engine began to shake violently and had a very loud slap/knock. A reduction of power reduced the noise and vibrations. A power setting of about 1300 RPM or more the engine began to shake so violently that it was a concern in my opinion to the airframe. I decide to divert to a nearby airport because it was closer; had more favorable crosswind; and the terrain was lower. The engine instruments and tachometer were normal the tachometer was not indicating a change in or unstable RPM. After landing I check oil level it was still at 6 quarts during the preflight the oil was at about 6.75-7 quarts. There was no obvious external damage to the aircraft.

Original NASA ASRS Text

Title: A PA28-181 Instructor reported a violent; very loud slapping noise during descent with the engine instruments indicating normally so he diverted to a nearby airport for a safe landing.

Narrative: After nearly 2 hours of flight during the descent to my home airport the engine began to shake violently and had a very loud slap/knock. A reduction of power reduced the noise and vibrations. A power setting of about 1300 RPM or more the engine began to shake so violently that it was a concern in my opinion to the airframe. I decide to divert to a nearby airport because it was closer; had more favorable crosswind; and the terrain was lower. The engine instruments and tachometer were normal the tachometer was not indicating a change in or unstable RPM. After landing I check oil level it was still at 6 quarts during the preflight the oil was at about 6.75-7 quarts. There was no obvious external damage to the aircraft.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1262941
Time
Date	201505
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Flight Phase	Descent
Component
Aircraft Component	Engine
Person 1
Function	Instructor
Qualification	Flight Crew Commercial
Events
Anomaly	Aircraft Equipment Problem Critical

A PA28-181 instructor reported a violent; very loud slapping noise during descent with the engine instruments indicating normally so he diverted to a nearby airport for a safe landing.