My XJR has started to 'growl' when engine reaches 250000 revs.
any ideas ?

 

I'm assuming you meant 2500. If it's noisy at idle I'd suspect the coupler, if it's noisy at speed (2500). I'd be thinking bearings. Take the belt off the upper pulley and spin it through by hand to see is there is any "feeling" of bad bearings. Also don't rule out a bad idler/tensioner pulley bearing, easy to check with the s/c belt off. I just replaced those bearings on mine when I last had the s/c off for good measure. Also if you take the s/c belt off completely start the car and see if it is indeed gone (i.e. Ensuring it is a S/C issue)

 

I am shocked that is your only complaint at 250,000 rpm. First thing I would look at is the oil. Have you changed it? If not I would buy myself 2 Gm supercharger oil containers and a mity vac and change it. Not saying that will solve a bad bearing but it will at least rule out nasty oil or low oil.

 

Bit of a typo. meant 2500rpm.
Just realised that the noise is only heard when cruising in higher gears.
Will try changing oil as suggested. Thanks for the info.

 

I use a mitty vac attached to a piece of brake hardline that I feed in the fill. Park a bit downhill to empty as much as you can(or jack the rear up. Mine was nasty when I did it around 70k. Need to do it again.