Tire thump/bump
 

I recently purchased a 2007 C230. I took it down to my full service automatic carwash. It has a track system that pulls the car through the carwash while you sit inside with the car in neutral. I have now been through it twice and I feel a a slight thumping/bumping as the tires are pushed on the track system. I feel one "thump/bump" with each complete rotation of the tire. The track runs on the driver (left) side tires. I have been through the wash with many other vehicles and have never felt it. Does anyone know what the thump is?