It happens when you reverse out the driveway? Might be the suspension moving due to force in the opposite-than-usual direction. Get the top mounts for the front struts checked out. With age/use the metal and rubber seperates, giving play under heavy load in the strut top mount. To replace it the whole strut/spring needs to be taken out. You might be able to check it by popping the hood and removing the plug over the strut fixing bolt. You might see the rubber seperating from the metal. Sometimes it can be made better without removal by fitting a washer cut to the right size to give more thread for the nut to be tightened, but I do not know exactly how. The local shock shop did that for me on a 760, with minor improvement. Might also be the strut itself.

Have fun...