Buzzing sound

There is a faint buzzing sound coming from the dashboard area in my car. The power steering fluid was low, so I topped it off with the recommended fluid. That did not solve the problem. I have not changed the fluid. So now I am stuck. All the other fluids are newly changed and full. It is only audible when the car is moving. The steering wheel has also become a lot easier to move. It almost feels lose. But it could be that I have been used to a tougher steering wheel because of the power steering fluid being low. I have read that it could be transmission, but I was just at my mechanic and he said it was fine. I also read that it could be wheel bearing, or a failing power steering pump. I do know that my tires and wheel are misaligned, could this be creating the problem? Could it be that I need to flush out my power steering fluid? Is it safe to drive？