I have 1998 Trans Am, A4 lightly modded. About a month ago I was leaving for work one morning and I put it in reverse, but soon as I put it in the gear the whole car started vibrating. I just blew it off until i backed out to the end of the driveway and I noticed what I thought was a "clunking" noise but now it sounds more like something's hitting against the bottom of the car. "Drive" is fine and the car changes gears fine while driving. Any help or suggested would be appreciated. Thanks.