My little Ford Fiesta Freestyle recently passed it MOT and is in pretty good nick

However

I have noticed recently that it makes a noise when the acceleration increases. The sound can be best described as a whirring or propeller sound (not the best description but you might get an idea). This sounds increases with the speed but being an old car its at its best at 70 mph.

Everything seems to be fine in terms of the coolant, oil, etc.

I am useless with cars and haven't got a clue and scanning through the forums.

It it s not incredibly loud and pretty subtle but the more the accelartion goes up the noiser it can get.

Any ideas?