94 Altima Clunking Sound...NEED HELP!!!

I have a 94 Altima GXE 2.4L Automatic. While backing in my driveway this morning I noticed a clunking/thumping sound coming from the front end. I stopped & put it in drive and drove forward but no sound. Started backing up again & there it was once again. It's not constant, its makes the sound every couple feet, and only when backing up. Not sure what the problem is & I can't afford to take it to the shop to find the problem, so someone PLEASE HELP.