Booming noise from silencer

[GATV6S]

A couple of months ago whilst returning from a day in Nottingham, I noticed that the exhaust note had changed from the usual rasp to more of a booming noise under acceleration.
Took the car into my local Jaguar services and Jaguar authorised replacement of the rear half of the system.
All has gone well for a few weeks until a trip out found the noise returned on the new system.
It seems to happen when the system gets to running temperature and after slowing for a bit, it then starts booming when you raise the revs.
Just wondered if anyone else has had this fault.
My model is a Coupe S.
The garage confirm it is the silencer and are now asking Jaguar if they have any reports of this not yet published.