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Is this a loud whine which could be described as a grinding noise on deceleration or a distant whine which clears on deceleration?
This does actually sound normal for these transmissions. Bearing noise will make a grinding noise which changes between acceleration and deceleration so I'm confident there are no such issues with bearings. The whine you hear from 1st to 2nd is caused by the shape of the gear teeth. Helical cut gears are used on most gearboxes (Cut at an angle) This gives a much quieter gearbox, however helical cut gears compromise on strength of the teeth. Straight cut gears are very loud due to the meshing angle of the teeth, however they are very strong in comparison.The first gear set on your transmission uses helical cut gears as per all of the other gears, but due to the high loads on the first gear set and large amount of torque, as well as getting more use from pulling away etc. they have been cut at a slightly straighter angle which gives a much stronger first gear. Unfortunately as with most things mechanical, It is a compromise between refinement and strength, Bentley have got the balance as good as they can with the noise to strength ratio offering a very strong gearbox but this does naturally cause more noise.
The more use the gearbox gets it is normal for this noise to very gradually become more prominent due to the natural wear on the teeth, comparing one for e.g with 100k miles to a new one you would notice a difference, however this is also completely normal due to the natural wear of the meshing surfaces of the teeth. Another contributing factor is the quality of transmission oil. If the oil has degraded it can cause more noise, so regular servicing when required is important but these are all things that make a "normal" sound more audible rather than causing a problem.
There are additives, but with an Auto box like this, I would be inclined to avoid them where possible and opt for a good quality oil change instead (if its not been done for a while) Otherwise Lucas sell an additive for gearboxes which will help reduce meshing noise of the gears. If you are to add this, i recommend an oil change at the same time