X5 E53 Screeching Noise!

[sja]

I have a 53 plate 3.0d X5. When I "floor" its it makes a screeching noise, like a slipping belt (if that makes sense). Its been making this noise for some time now but as I never really accelerate hard it hasnt been much of an issue. However I would like it sorted now. So before I take it to a dealer / Indy I would like to see if any of you could advise suggestions as to what it is? Looking at some posts on x5drivers it was suggested it could be gearbox (just had oil changed made no difference), or the exhaust manifold gasket being split?

When not in gear and being revved it doesnt make the noise. Its only seems to appear under hard acceleration or when under load, i.e going up a hill.

Thanks

Steve

[sja]

It was the Turbo apparently. Indy qouted me £1200 inc new breather hoses.