Air carrier First Officer reported the flight returned to the departure airport due to an unexplained banging noise from underneath the cabin floor.

Narrative

During climb above FL230; we were alerted by the flight attendants in the forward galley that a loud continuous banging was coming from underneath the floor. The flight attendant said the noise sounded like a person pounding on the ceiling of the forward baggage compartment. The Captain immediately called ZZZ operations and dispatch. Not long after ZZZ opts began the process of getting a head count of the ramp personnel that worked our flight; they recommended we return back to ZZZ. The Captain and I agreed due to the abnormal nature of the banging noise; a return back to ZZZ was the safest course of action. We coordinated a return back to ZZZ with ATC; and dispatch. I referenced the FOM; and the Overweight Landing section in the FM; as our return would result in a landing over MLW. As we were initiating the return back to ZZZ; the banging noise stopped and did not return for the remainder of the flight. We needed some time to set up the diversion back to ZZZ; so we requested a decent to an altitude below FL180 and a hold from ATC. ATC held us at ZZZZZ on the ZZZZZ arrival; and I advised ATC we would be maintaining a higher speed than normal in the hold to burn more fuel to get our weight close to MLW. After we reviewed the FOM/FM and we were set up for the diversion; we continued the arrival and approach to Runway XXR. The landing was normal; with a smooth touchdown and normal braking; followed by a normal taxi to the gate. After shutdown; we made the ELB entries for the banging noise and the overweight landing. Ramp personnel inspected the cargo compartments; and there was no person trapped inside. To my knowledge the cause of the loud banging noise is still unknown.