If you set the emergency brake(s) .... they could make a sound like that as they release...??
Sometimes, on some cars, the ABS does a "self test" on startup ..... which will make a "clunk" sound...??
It could be the rear calipers are cold (stiff) and abruptly release the pads ...??
It could be that "sheet metal rear panel" shifting making a sound that some folks experience ...??
It could be that the differential is just cold ... ??
Maybe the rear axle universals are worn ... ??
If it continues, you may wish to take it into a shop for a look see.....
humfrz