As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, WHILE PERFORMING A SLIGHT LEFT TURN, JUST PRIOR TO LEVELING OFF. HEARD A LOUD "THUD" FROM UNDERNEATH ACFT IN THE VICINITY OF PILOTS SEAT. IMMEDIATELY RETURNED TO DEPARTURE AND LANDED WITHOUT INCIDENT. ACFT GROUNDED. MX SUSPECTS THAT THE SOURCE OF THIS PROBLEM IS A FLEXING BELLY ACCESS PANEL (NON-STRUCTURAL).