I own a 2003 Foreman ES 450 that I bought new. It has about 3,000 miles on it. I change the oil with synthetics at least once a season. Begining at 30 mPh I begin to hear a mild rattling noise. This has been the case since I bought it. Any thoughts on what this noise is? I also put a set of mud tires on recently. It seems like the gas mileage has dropped 50% since the installation. Is that normal?