As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE PILOT REPORTED SHORTLY AFTER TAKEOFF, HEARD A LOUD "THUMP" COMING FROM THE FLOOR IN THE VICINITY OF THE PILOTS STATION. THE WERE NO FLUCTUATIONS ON ANY OF THE GAUGES AND NO ABNORMAL FEELING IN THE CONTROLS. ABORTED THE MISSION AND RETURNED TO THE BASE. THE CAUSE OF THIS THUMP IS STILL IN INVESTIGATION. THE ACFT REMAINS OUT OF SERVICE AT THIS TIME.