Maybe it was an elephant gun?? Or an elephant train on a siding? Or the sound of a giant anchovy hitting the pavement!

Seriously, no one can say for sure here on the website, just endless fun speculating. I have done a lot of shooting in my day and heard gunshots in all kinds of situations. So much depends on the terrain, wind conditions, buildings present, humidity. The same gun can sound like a "ladyfinger" firecracker or a peal of thunder. Same for a backfire. Given your situation you can rest almost entirely assured that it was a backfire.