'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!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.