Driving me crazy! Hollow tone under light throttle 2.0 tdi CR

Hey Folks,

Really hoping someone can help!!!

I've picked up a 2.0TDi CR, and its got the most annoying problem in the world.

When touching throttle lightly, maybe 5% on the car emits a hollow, pretty constant tone. Does this in all gears, but the tone can be a different note, and its most notable at 42mph in 4th or 52mph in 5th. Although similar they are different points in the rev range.

There is also a tone when revving at standstill, lower in pitch, which I think its the same thing.

Only really does it when the engines properly warmed up. If you up the throttle a bit it goes away.

Might have to ditch the car if I can't get to the bottom of it, its that annoying. Turning fans onto full almost covers the sound.

Its just had the turbo rebuilt by the garage too - was all coked up. Cant see it being turbo related now.

I'm thinking possibly fuel pump? Could it be the a/c compressor? (there is an a/c leak + its not working, although compressor runs).

Any ideas welcome

Thanks

ADMIN: I posted this in the faq section by mistake, maybe delete that thread? Sorry - ta!

Incase anyone else gets similar issues it was drivetrain - diff or gearbox, didn't get in diagnosed properly, tried a gearbox oil charge, then got shot of the car!