Air Carrier Captain reported during taxi for takeoff hearing multiple loud bang sounds resulting in the flight crew not accepting the takeoff clearance and returning to the ramp where it was discovered that a cargo bin was not secured.

Narrative

Flight XXX ZZZ-ZZZ1. Blocked out XA:09Z; approx. 9 minutes late due to loading delay. On taxi out of [the] ramp; crew (CA; FO; CHK; and Jumpseater) heard a bang from near the L1 door/door to from the flight deck to the main cargo deck. Stopped on taxiway after noticing the door from flight deck to main cargo deck hag swung open. Jumpseater closed door with a/c stopped and parking brake set on taxiway. Continued taxiing and open receiving takeoff clearance; taxied onto runway XX for departure. Making the turn to line up on runway there was another loud band noise from what sounded like A1 position. Told FO to tell tower cancel take off clearance. Takeoff clearance cancelled (no takeoff thrust applied; takeoff roll not started; no high speed abort-but takeoff clearance was received and cancelled; hence the selection of aborted takeoff). Taxied back to [the] ramp; waited about 5 minutes to be parked and shut down. Explained the noises to loading crew and they began investigation. They found the can in A1 had been loaded improperly with the single; heavy box of freight inside the can not being secured and therefore sliding around freely. They began to offload; but storms came in with lightening and the ramp was shut down. Approx 3 hours passed with the ramp shut down before the ground crew returned. They decided to just pull the A1 can and reload everything else to try to get us out before the next batch of storms came into ZZZ. Completed another cross check; got new flight plan and numbers from flight control; verified the A1 can was not loaded; and departed again for ZZZ1 with no further issue. The whole thing ended up causing what amounted to about a 4 hour delay.