Had mine fixed on the o/s front the other week - sounded like a rumbling noise - only when the car was moving - thought the gearbox had gone, but it wasnt that thank god...the noise got progressively worse...
When it went for repair, the mechanic said the bearing had cracked and was close to seizing up!
Try driving the car with all the heaters, stereo off etc so you can hear it, but be careful - if it is the bearing, dont drive it for ages like that