Joe you can't discount the gearbox even if you still get the noise when in neutral.
Simon.h had a similar noise and this what he found:
'The only way I found out 100% it was the gearbox, was to jack the front of the car off the ground and safety sit it on blocks of timber under the chassis so the engine could be taken up through the gears. With the car in 5th gear and doing 50 mph with both wheels spinning (with my son at the wheel and me laying under the car) I got him to drop it into neutral and turn engine off so wheels spin from 50 mph to 0 mph without any noise from engine.
And the noise the gear was unbelievable!'
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