Why is starter motor making a high pitch squealing noise when turning key but not starting?

I fitted a new starter motor about 3 weeks ago, but before I replaced starter motor the old starter made this very same noise, and I was told it was the RINGEAR, which apparently is something too do with the starter motor, now if this is the ringear is this an expensive job or shall I just get rid, or is it a simple job, I just don't know what too do now because as soon as turn the engine off it won't start without jacking the car up each time and knocking the starter motor with a hammer.