Weird grinding sound

[Balkanac]

Hello all,

I have an e53 X5 3L and i have noticed this weird grinding sound coming from the car when driving. I can't hear it when driving, but if i drive past a wall or a parked car with one of the front windows open it sounds like grinding noise coming from the wheels. Same noise from both sides.
Eventually the front pads got too thin and the noise stopped. I changed the pads thinking it must be them, but the noise returned with new pads. I also had the front calipers rebuilt including new pistons but the noise is still there. I think it gets quieter when braking but i cant be sure

Any ideas anyone??