I have had the 2009 YZ450F for about 1 year and have put maybe 25 hours on it. I have not raced the bike or even rode it extremely hard. I change the oil and clean the air filter after every other ride. Just this spring I noticed a hollow sound coming from the motor when it is is idle or low rev. It almost sounds like I can hear a gentle knock in there and I am wondering if anyone else has this problem or if they would know what it is???