Sport 2006 S/C Sport Whine

Hi, Long time JLR machosist currently with LR3, Freelander 2, Jag XKR vert and a new shape Evoque (for the wife) on the drive, found a couple of grand lurking unspent, so bought an early Supercharged Sport. I'd have liked an L322 but all the ones I saw (07,08) were absolutely rotten at the rear, so not ever having a Mk1 Sport I thought I'd give it a go. Car had faults when I bought it but most soon sorted with a repair to the amp under the drivers seat and some new ARB bushes. Love the car especially the handling but I've a whine on the rear at speed I cant identify.
It's very audible at 65mph but goes at 80, its definitely louder when you apply throttle and goes away when you lift off. I've had the diff oil changed and the oil that came out of it was clean and sparkle free, I suspect the diff but wondered if there was something I might be missing...do the transfer boxes whine? is there any test I can do other than hanging under it on the motorway?
Assuming it is the rear diff, does anyone know whether they are all the same (2.7,3.0 3.6) or is the Supercharged diff strengthened/different in some way to take the extra power.
Any advice would be most welcome.
Cheers Ian

Check the rear prop centre bearing, the rear brake calipers for binding, park brake shoe adjustment and rear differential input bearing (disconnect the propshaft first for free play. Also check the tyres - are they 'ditchfinders'? I've had a lot of drivetrain noises in the past where the root cause was the tyres or damaged / repaired wheel rims - sometimes the only way to prove it to the owner was to temporarily swap with the wheels from a sales car...
The differentials are the same, only the manual 'box has different ratios. The diesels actually put more torque through the drivetrain than the SC.
Does it have an open or locking rear differential? Transfer boxes can indeed cause transmission noise, usually most evident when turning at speed.