KLE carburetor problem
 

Hi everyone, I'm Alex from Rome, Italy, new to this forum and to this great bike. I've noticed a strange noise coming from below the seat, so I decided to take a look and I've seen that the entire carb (only once in a while and expecially when the engine is cold and running at the idle) shake visibly. Just a single strong shake (STOK!) which may repeat after a second or stop for several minutes. I've never seen something like this before and I was wondering if it may be related to the cooling system and the valve which allows the coolant in/out the hoses.
Any idea or suggestion?

many thanks,
lamps!