Power Antenna Goes Up/Down but grinds

Hey just recently I just intalled my new Alpine CDA-9811 deck and wired up the blue wires for the power antenna so for that first day it worked fine. The only concern is that when it goes to radio mode the antenna works fine going up but when it does down to about reaching the end I hear a grinding noise WHEN IT REACHES THE VERY END ONLY and the antenna will do down with only about a 1/2 inch of antenna sticking out left. What may be the problem?

I read the previous posts and so far I guess buying a new antenna mast is only if your antenna is stuck up or will not go up. But my antenna goes up and down.

Thanks!

the nylon piece that raises and lowers the antenna (inside the shaft), may be broken

better off than me, my power antenna dosent go up or down. the antenna motor comes on and starts making loud noise but dosent pull it down.