similar complaint from other owner, I have noticed a very consistant knock knock knock noise coming from the right side rear wheel area. Ford dealer service manager said its a normal noise and most likely the wind hitting the license plate. I didn't agree and while they had my mustang in a lift I went to speak with a technician and he noticed the problem right away. The right side exhaust was missing bolts and he took care of the problem. 5 months later I upgraded to flowmaster and I noticed that the left exhaust was missing the clamp exhaust. The dealer took care of the right side but didn't even bother to check the left side.