I am now having this problem. It started as a low noise that is proportional to the speed of the car, not proportional to the revs of the engine. It has got worse and worse over a long period of about a year on my 2000 focus. On a trip to wales last weekend it get very loud and it was a bit worrying - now I see that it is likely wheel bearing noise and could have been dangerous.
Taking it in to the garage on this friday.