Lot more burbling and spitting from exhaust X308 XJR

[Zippyworld]

Recently I have noticed my 98 XJR has changed its sound.
The car has a CATS back stainless exhaust and its been on for 5 years and done 15k miles.

It has always had an occasional pop which is fine, however now its more often and it sounds just a bit "blowy" and spluttery, performance seems fine so I am scratching my head a little.....

Anyone got any suggestions please ?