What was found
DURING OPERATIONAL CHECK FLIGHT FELT A VIBRATION AT 20,000 FT ASL, AIRSPEED 230 KIAS CONDITION LEVERS 900. VIBRATION ENDED AT AIRSPEED 246 KIAS CONDITION LEVERS 850. EVENT MARKER PUSHED. ALL ENGINE SYSTEMS WERE DISPLAYING NORMAL INDICATIONS. FELT LIKE AIRFRAME VIBRATION SIMILAR TO DRIVING A CAR ON RUMBLE STRIPS. AIRCRAFT LANDED SAFELY AND WAS REMOVED FROM SERVICE. CURRENTLY OUT OF SERVICE FOR REPARS.