What was found
DURING FORWARD FLIGHT, CREW EXPERIENCED RAPID AND ABRUPT VERTICAL UP/DOWN SHIFT. WINDS WERE CALM. PIC SLOWED AIRSPEED TO APPROXIMATELY 70-80 KNOTS AND EXPERIENCED A SECOND OCCURRENCE. CREW DECIDED TO CONDUCT A PRECAUTIONARY LINING AT KRDU APPROXIMATELY 4 MILES TO OUR TWELVE O'CLOCK. THE NEXT OCCURRENCE WAS EVEN MORE PRONOUNCED AND THE PIC DECIDED TO LAND AT THE NEAREST SUITABLE LANDING AREA. AT APPROXIMATELY 80FT AGL ON SHORT FINAL TO THE UNIMPROVED LZ, ANOTHER ABRUPT VERTICAL SHAKE WAS FELT BY THE CREW AND IN THE FLIGHT CONTROLS. PIC LANDED THE AIRCRAFT WITHOUT INCIDENT IN A HOUSING SUBDIVISION.