I have noticed recently that, when driving with the windows down, I can hear a clicking/tapping sound from the rear of the car. At first I thought it was a stone caught in one of the tyres 'tapping' against the ground. I looked but there were no stones. Yesterday, while cleaning the car, I noticed that there was something stuck to the back of the spokes of one of my wheels. The stuff was coming away from the wheels and made a similar sound to the one I heard when it was 'brushed against' at the edge. Could it be that they stick some kind of protective plastic to the wheels when they are new and it was never removed?