This is odd, it is winter here in Ohio so after I have started the Saab and driven anywhere between 5 and 10 minutes, there is a thud sound in the roof as though a rock had hit the car. My guess is the metal in the roof is expanding due to the car heating up ????
Do you have a sunroof? If so, have that checked.
Of course, you shouldn't rule out envious bystanders throwing rocks / snowballs at your car...
Hope they didn't use surplus Cavalier parts, my Cavalier does that. I thought it was stuff falling from trees till I noticed it happened in wide-open areas also. My guess is that your guess is correct--I have no solution though.
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