Fivehunnit

so I have recently noticed a distant 'woooooooooo' sound at certain speeds (possibly different in different gears also?) it's not the tyres, they are a bit noisy but different, this noise disappears if I'm going around 30mph and stick the car in 'N' (I know that's not a good idea, so only done it once or twice to see) don't know if that's ruled anything out? It's not loud but definitely there, the wife doesn't hear it apparently :/
could it be gearbox pump related? Heard they can be a bit of a common problem also? Thanks