I'm testing the beta2 version now on a regular basis. I'm trying to look through the graphical issues (support for an ATI Radeon on windows isn't great currently).
One thing I noticed was that I heard the collision sound when no other karts or objects were present. Investigation this further I noticed that this happened usually on a dip or bump or change of road surface. Therefore I think it can only mean one thing: A collision with the road surface. It seems that the speed also dips when the kart collides with the road surface (I can't actually read the speed since the gauge in the bottom right doesn't work).
Has anyone else noticed this as well? And if so, should I log a ticket?