old school drag racers usedto put bb's or ball berrings in the tires to 'help traction' by keeping weight on the tread where it makes contact with the ground...
sure your not hearing the berrings rollin around in the carrier?? if the berrings are junk the balls will fall out and roll around
could be small pebbles mine do the same thing since i ride sand pits
if your gettin stuff inside your tires you have prblems