BMW 3 Series differencial and chassis

i recently bought this car and was told it was an intermittent clicking in the driveshaft when the noise became worse i took it to the garage only too find out that my diff was being held by 3 bolts instead of 4 and tha it was comin away from the chassis iv had the car repaired for 1 and a bit weeks but the same noise came bk i had the work checked and the welding is fine and the weld is still holding perfectly the noise went but now its back with a vengence and i was wondering if anyone else could think of anything else it could be as the garage cant find what this noise is