I have a 2000 Mustang GT manual tranny and 145,000 miles. Recently I got the A/C up and working again due to a low pressure switch being faulty. Since then, anytime I am coming to a complete stop a loud shaking noise goes off in the rear of the car. I've been able to tell that it happens the same time my RPMs return to idle. This does not happen, however, when the A/C is off. Also, when I'm on the highway theres is a loud vibration when at 64-66 MPH. I have no idea what causes that? Any healp will be greatly appreciated.