Interesting that this happened when supercharging. If it was a sheet metal buckling I would expect sounds like these to happen as the chassis flexes. Though its still possible that the sheet metal is sprung and stays that way during driving, but pops as it relaxes.
The fact that it happens while stopped pretty much eliminates the sound coming from anything rolling around loose and banging into something. I guess its possible it could be related to the battery cooling system, since that is certainly active while supercharging.
Also possible its something else entirely. You mention it happens at random times, does it happen more while cruising, driving hard, cornering, accelerating, speedbumps/uneven surfaces or braking?