08 MCS making a popping sound (sound seems to come from beneath car to the right rear) after I turn it off; this has only occurred twice (3700 miles)....seems to have something to do with hot exhaust system cooling but wanted to get some input from you Mini experts. What do ya think?

 

When this has come up in the past, people seem to agree that this is the gas tank venting system burping stored up pressure.

It seems to happen more often to me when I've got a full or nearly-full tank.