I know of this problem I have experienced it with my own vehicle your dealership should know all about it. It is the bulkhead behind the back seats. In each corner two pieces of metal overlap resulting in a sharp click. I took it to the dealership thinking it was a cable loose somewhere in the boot. The engineer informed me what the problem was and the remedy which was to pack the metal with grease. This they did and have had no problems since. The time in garage was about 20 mins.
my 86 is 3 years old and I presume it was the earlier models which had this minor problem.
Good Luck
