My neighbour drives a small motorcycle that sounds funny in my ears. I don’t know about motrcycles (only earlier on mopeds) but I’ve never noticed such a sound before. It’s some small cross-maschine. When the engine is running you hear the normal engine sound and additionally a kind of squeak as if you push a six-pack Selters(with foil) over a laminate floor. It’s in the beat of the rather deep engine sound. What can that be? Is that normal?