ok this may be a bit of an odd one but i'll give it a shot just in case someone else has had the same thing.
on my Meg2 GT150 DCi i've got a sort of mid-toned oooooohhh sound (best description i can give ) that kicks in about 30-40mph and it get slightly higher pitched and louder as the car gets faster.
The sound only changes for speed and not when changing gear or revs getting high. I realise this could be a multitude of things ranging from a dodgy tyre to something mechanical. Just thought i'd ask in case it happened to anyone else.
I'll need to take the car into a garage soon as it's gradually getting worse(louder).