Funny..I was going to post about this exact thing today. Ours is sometimes rather loud like a tennisball hitting the back of the car. Other times ..yup...acorn hitting it. I dont think its the expansion thing. I experience it daily...first drive, 2nd 3rd, no matter...happens like clockwork about 1/8th of a mile into a drive. No matter if sitting outside in the sun before, in a garage, raining, etc....always happens.
Strange.
Strange.