When it started on my car, it gradually got worse, even at lower road speeds. If it's on the n/s rear wheel, when you go round some left-hand bends, the noise should reduce (I also used to try weaving from outside lane to inside lane on an empty motorway, if you try this yourself and come a cropper, I take no responsibility of course). It's usually not an MOT failure because the bearing is usually not loose. The bearing is cheaper and easier to replace on the saloon than on the Tourer.