Rear axle bushes are a common prob with mk4's and result in a nasty banging sound when you go over any bumps, i'd bet this is your problem. Do a search on this and you should come up with a few posts. Gti rear axle bushes are oil filled and will require your rear axle to be replaced / part exchanged.......... However you can just replace them with solid bushes from a 1.6 or other non Gti without replacing the rear axle. Its a pig of a job to do though so expect to be paying for about 4 hours labour.
HTH