My ex hubby has an BMW 318i auto and it makes this whurring noise all the time, worse when accelerating.

Someone said to him it might be low gearbox oil level, does this make any sense or are there many other things it could be?

I think it behaves the way it should when I've driven it (apart from the annoying noise of course) but then I've never driven an automatic before so what do I know right?

Any advise would be great thanks!!