I am really confused right now. I just got a new exhaust on my '89 and a day later i am driving down the street and all the sudden it sounds like my tires are rubbing on something, or at least that is what it sounded like, then it felt like i had a flat, really hard bumping. I pulled over and everything looks fine. It only happens in reverse and when i let off the gas in drive, the noise and vibration will go away when i accelerate and come back the second i take my foot off the gas. It runs perfect at idle also. Any ideas?