Make sure the bottom arm is not rubbing against the frame.
This happened to me once, after a horrific crash.
The frame seemed straight, but the only way I could resolve it was to run the gearbox without oil until it locked up and replaced it with another unit.
Problem solved! (really...)
It was hard as hell to document. This is the best I could do:
http://www.youtube.com/watch?v=tH1NVdZdkbc
It is almost easier to hear it than it is to see it. You could see the paint being scraped off, though.
