The strange sounds arenĀ“t really in the engine, they come from the exhaust where there is a special device which takes air from before the air box and places them after the combustion chamber to help ignite the unburnt fuel before it goes through the catalytic convertor as this would damage it, it also reduces emissions. This noise is apparent on most engines of this size. I used to have a 125 with the same thing. If you look at the engine you will notice a tube that goes down the left hand side and into the exhaust under the engine.