The grinding noise is an indication that the lens motor is damaged. As mentioned in the comments, manually adjusting the focus while the AF is switched on can damage the AF motor. Certain lenses (usually the expensive ones) have full time manual focus where you can have the AF on and use the manual focus ring without worry.
I have not handled a lot of lenses, but from what I can tell if the AF moves the focus ring or if you can only move the focus ring so far in either direction then it does not have full time manual focus. Conversely, if the AF does not move the focus ring and/or the focus ring can be rotated infinitely in either direction then it does have full time manual control.
Edit: the AF motor does make noise during normal operation, but I imagine that you know what this sounds like by now. This is not the same as a grinding noise that could occur if you cause damage to the AF motor's gears.