i think when mine did that it was the motor side of the carb, and the metal strap tightener came loose i couldnt tell by looking i had to tighten it up good and it want away. it IS an AIR LEAK. thats fact, but were can be fun to figure out. try on the inside of your airbox and make sure every tightener is tight and on the carb and the reeds. the reeds could need some sealer or a new gasket, but its definatly an air leak.