B5.5 1.9 TDi centre exhaust silencer rattle

[Brind]

I have a rattle that resonated at exactly 900RPM, I asked a mechanic mate to have a look under the car for the cause when he had it up in the air on a lift whilst he was doing the CV boot for me. He said he had a poke around and thought he had sorted it but it was still the same when I got the car back. If anything, it's worse now.

Annoyed by the buzz, I backed the car up on my own ramps to have a look under the car myself with the engine running and located it to be the rear centre silencer.
If I push it up, grab it and pull it down, I can get it to stop. If I push it to the side, I can make it rattle very loudly.
Trouble is, what ever I do, I can't stop it without hanging off it or forcing it to stop with my hands. I can also stop the sound by pushing up on the joining piece between the silencer and the middle of the car.

I thought I'd get some exhaust wrap and wrap it into a ball and wedge it between the silencer and the heat shield on the car, this sort of helped with the buzz to a slight degree but now transfers the noise to vibration too.

The silencer itself doesn't appear to have anything loose inside of it, it also doesn't appear to be touching the chassis in any way either. Yet it's making a hell of a racket at low revs.

Has anyone else come upon this problem and possibly sorted it? I am 99.9% sure it's not a heat shield. Everything is tight and secure. Even the silencer is hanging off two rubber brackets which are in good condition.

I'm at a loss with this. There's got to be a way of stopping the buzz and rattle.

SORTED: I replaced both exhaust rubbers holding the centre box at the rear. They looked in good condition, but I guess the weight was enough to stretch these rubbers enough to not be as sturdy as they once were. And since there's nothing holding the pipe all the way from the engine area all the way back to this box, this is probably why.
I replaced them with uprated mounts made from polyurethane from Powerflex. These don't stretch like rubber but still have bounce in them.
Before I got them I did cable tie the original rubber and this pretty much worked but thought I'd do it properly and get something decent. I have put a chunky cable tie around each one just for peace of mind to prevent it stretching, not that it should though.
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