As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE FLIGHT, CREW FELT A SUDDEN HIGH FREQUENCY VIBRATION IN THE AIRFRAME AND HEARD A CORRESPONDING NOISE. NO ABNORMAL COCKPIT INDICATIONS WERE NOTED. CREW PROCEEDED TO LAND AT BASE. UPON INSPECTION, MX FOUND NR1 INPUT DRIVESHAFT FWD COUPLING DISTORTED AND CRACKED. FURTHER INVESTIGATION FOUND NUMEROUS BREAKS IN THE COUPLING LAMINATES.