Anyone else have this issue...
Sounds like a sead or nut falling off a tree and hitting the car. It happens once in awhile and can be with the car off or sitting at a light with the radio off.

 

It happens with my car too. It comes from the resonator under the car. It actually is quite annoying and the more aggressive you drive the more frequently it happens. I asked the dealer what the cause was but they said they did not hear anything, WTF!

 

I too hear this on rare occassion (yet I've never heard it at a stop) but have yet to isolate the cause. I assume it really is some road debris yet I've never been able to see it or any "damage" (e.g. stone chips, dings, etc.)