Mouser

Seems to be coming from the engine bay area almost like another engine running but at a higher pitch at speeds over 40mph and in 5th gear(top). Car has only done 22k miles and is not unpleasant like a squealing sound, it is almost like the car needs to be changing into a higher gear or as if it has lost some sound deadening. Any guidance would be appreciated. I only noticed the noise when the radio lost reception in some hills and was turned off so perhaps it has been there all the time! I suspect the transmission but on such a low mileage I would doubt it. The car sailed through the first MOT.