Strange cause for polterns in the rear

Moin Leude,
Nice forum here, that has to say. I have been registered since yesterday (5.8.) and have quickly received refreshing answers to my questions. Very nice, so far.
But now I want to give something about the topic "klappern in the rear area best:
When I got the V8 he made ugly noises at the back right. I changed the ball head of the auxiliary handlebar (with which you adjust the fall). That thing was completely knocked out. The cacophony in the rear was actually poorer on it. But there was still a clear beating to be felt when you, for example, were not able to get a good start. It's over a curb edge. After further examination I renewed the front bearings of the lower handlebars and the hard beating was gone. Nevertheless, there was still a slight blotch to hear. But everything was fixed! I have been riding with mooring levers and crowbars but nowhere was there any game in it. Shock absorber test: "Not broken but not good anymore", was the little helpful statement of the tyre man who made the test. All right, new rear shock absorbers built in and look, the noise has changed its sound again! Now there was only a quiet, hollow knock to hear, a little bit as if a little stone were curling in a tin can. Slowly I started to push despair...But since I think that there can only be one winner in the fight against technology, I have continued to seek. As the owner of a Citroen XM V6 with hydraulics and all pipapo I am reasonably steeled.
Anyway, the car is up again and from the bottom it's hit again and trampled and, what do I have to hear, and that's from the tank.....
I then opened the tank and found a tool in it (IMHO the one with which you build the gasoline pump)! A round part with a handle on it! There's been silence in the back ever since! If anyone ever needs a tool like this, I'd love to lend it.

Best regards from Nordfriesland
Tim