I noticed this when following Matt's cgt but not Fayence. I was behind Fayence on the freeway and noticed nothing, but when I was behind Matt when we were doing some spirited driving through newport I heard a few get kicked up onto the car. I guess it depends on what road you are on though. Maybe less rocks on the freeway then the regular street?

It is definitely from the ps2's though. I put ps2's on my m3 and they are not even close to that size but they still kicked up some pebbles here and there.