My dad and I were riding over the weekend and his rincon started making this horible sound from the rear of the bike. I thought it was just mud/dust around the roater for the rear break. however, once we got home and he washed it, it was still making this noise...although much softer. It is not a constent noise but rather "pulses" of sound that increase and decrease in volume like a sin wave. Is there some type of bearing or something that could be going out??? it only has 400 miles on it...