Loud Pop When Turning Off Van

A couple of seconds after I turn off my 2005 Saturn Relay I hear a loud pop underneath and towards the rear. It kind of sounds like comething hitting a tin can. The dealership is telling me I have to replace the differential for $3,500. I think this is a bunch of baloney. Could it be the muffler making that noise? If it’s the differential does it just need its fluid replaced or is a whole new one in order? Thanks

Sure! Exhaust components can make noise after turning off a hot engine. This can be a popping, crackling, or sound like a lot of little hammers are hitting a pipe.

It’s not the differential as the vehicle isn’t in motion.

Tester