It sounds a bit like an rc car motor. Like a little brrrr but kinda high pitched. It is a low version of what a rabbit squeak sounds like but much more like purring. He does it when he wants something.
Musti used to snore when he slept, sounded a lil bit like a car engine too, like snnnr snnnrh, was really entertaining to listen to but never got it on video-the boy would know what was up and stop snoring immediately.