Sound from rear of car

I suppose it could be what PH2 said. I never in a million years would have guessed that, so I am not positive that is where the noise is coming from. I know it is not bump related or anything like that. It usually occurs within the first half mile of driving. And if you have ever run over a small piece of rubber such as from a tire and it bounces off the inner wheel well, that is exactly what it sounds like. And I am positive that is not the cause. It is too "regular" of a noise to be something I run over. Sounds like it is coming from the right rear of the car. I will look at the spoiler as PH2 mentioned, but even if that is the cause, I assume it would not be something you would be able to notice just by looking at it. Anyway, keep the ideas coming. THANKS

I can agree the sound being like an acorn hitting the back of the car. I couldn't initially use that analogy as there are not many acorns in Phoenix. A bit hot for those. Anyway, the car is parked in the garage every night so there may be a big temperature difference (hotter or colder) between the garage and the outside, so the thermal expansion theory would make some sense. As I said, it usually happens within a 1/4 mile of driving, but sometimes I go a mile or two before I hear it. So unless anyone has other ideas, I am going to agree with PH2 and assume it is the spoiler. THANKS