My 96 Cobra is has a bad pop/clank coming from the passenger's side on the rear. It only happens when I put the car into reverse and start backing up. It will pop once and that is it. Then when I put the car in first and go, it will pop once more. It doesn't happen every time, but quite often. I'm no mechanic, so I'll have to have someone else fix it. I need an idea of what it is/how much to repair, and also how big of a priority it is. I'm headed to the track this weekend, really wanted to run but looks like I may have to put it off. Any ideas? Thanks.