'94 325ic whines like a jet engine - Gearbox??

1994 325i convertible -
Had car for about 3 months. Love it but recently noticed a 'whining' noise whilst driving. Sounds like a high pitched jet engine noise. It's
not always there but probably around 70% of the time. Can be loud at times.
As i accelerate it gets a higher and higher pitch before dropping a tone as the car shifts up a gear ( auto), then starts all over again...
Doesn't whine when car is sitting in neutral when I rev so it's something thats happening whilst moving - bearings, diff or gearbox I think.. Appears as much as I can tell whilst driving to be coming from behind me - back middle somewhere??
Given it is affected by the gear change I am thinking gearbox??
What is it? Is it serious? Car appears to be driving fine!

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