new 7mm cub noise

Hey guys I just took my cub out for the frist trip and after the first day of riding the next morning its was really cold and i have a noise coming from the motor and its kind like a card in the spokes king of noise that goes faster and slower with the rpm range tell the motor heats up is this just the pistons needing to warm up or any ideas. It goes away as soon as the motor is good and warm and it runs like a monster and I road it the hole day with no proablems.