Have noticed that I hear a mild, barely audible "thump" when I put car into reverse when starting up. No other issues I know of- car seems to drive fine. Is this normal? Seems to coming from back of car. Thanks!

 

From back of car, it could be brakes sticking because of rust overnight. You can feel whenthey suddenly break loose.

Or maybe more likely, the exhaust pipe hitting something. The exhaust system hangers & brackets can get bent, moved around, or the rubber hangers stretch with age & heat. The system expands as the car warms up, so that could be why it only happens when you first start?