What was found
FOR SEVERAL MONTHS THE CREW DESCRIBED AN INTERMITTENT IN FLIGHT VIBRATION, STARTING TYPICALLY WHILE CLIMBING THROUGH 12K'-13K'. THE VIBRATION WOULD CONTINUE WHILE AT ALTITUDE & STOP ONCE BELOW 10K'. ENG SPEED THRUST LEVER POSITION DID NOT INFLUENCE THE VIBRATION. THEY REPORTED THAT THE VIBRATION WOULD GO AWAY IF THEY REDUCED THE AIRSPEED BELOW 250KTS. THE SOUND WAS REPORTED TO BE SIMILAR TO THAT OF A MOTORBOAT DRONING. EXTENSIVE TROUBLESHOOTING FINALLY FOUND THAT THE LT KRUEGER DE-ICE BOOT WAS LEAKING & WOULD NOT INFLATE CAUSING THE VIBRATION. THE BOOT WAS REPLACED & THERE HAVE BEEN NO FURTHER REPORTS OF THE VIBRATION OR UNUSUAL NOISE.