I think the answer I have seen that makes the most sense to me is that the sound is from the wheels running over the support rods in the track. As the hyfax wear, there is more pressure on these wheels against the track, so the noise gets louder. This is why when you go faster the noise gets less. The wheels simply don't have time to fall between each rod the way they do at slower speeds. It's just a guess, but a pretty educated guess. What I can't stand is when someone says something like, "You only hear it because the sled is so quiet" That is complete BS!!! This noise is louder than any 2 stroke engine I have ever owned!!