Remember that sound can translate through solid structures in some VERY suprising ways. If the car in front of you kicked up a rock and it hit the front cladding, it could sound like what you described.
I noticed something weird the other day, toatally unrelated but just as strange: I've got a rock chip in my rear window. It's right on the edge so I don't think it's repairable. It doesn't look like it'll spread but I'm going to take it to a glass shop to see what they think.