Robert Bouknight , Mar 20, 2013; 05:47 p.m.
Is the manual focus override stiff?
If the lens operates differently than it was when new, might very well be worth sending in. Any chance it has been dropped? If a bayonet lug were slightly bent in, difficulty or mounting would be higher, and something could have happened to the focusing mechanism to create the sound.
Check the bayonet for dirt or debris which could cause stiff mounting.