The closest I have ecer come to this is in a Z3. I have driven many Z3s and if they have 16" wheels they drive beautifully. However, when they have 17" wheels or higher, the steering has a mind of it's own!
Grumbling noise from bearings - it is unusual for both wheel bearings to "go" at the same time. Therefore the grumbling noise would be only when turning one way, or steady and constant all the time from one wheel, and you would feel it throught the steering wheel.
My conclusion is that it is tire noise and that you have oversized tires that are making the car "Tram line"