Just doing my part to help save another Cat.
I think what the difference is has something to do with the sway bar mount on the XJR which the std. XJ8 and VP don't have.
But my XJ Sport look same as the XJR..

 

I had a noise similar to that in my 98 XJ VDP. It turned out to be the aux heater pump. it comes on and off with the ac/heater, and is a constant speed pump, as far as I know. I can't say for sure if that is your problem, but it might be something to check out.

 

Thanks for that,

There's a wide array of areas the sound could be coming from but I'm 100% sure this was the diff. Ordered one from Texas and should be here soon.

They don't look too difficult to put in, they're the smaller units like in MB's