Ford 2003 Mustang Workshop Manual page 272

If metal has broken loose, the axle housing must be cleaned to remove particles that will cause
damage. At this time, any other new parts in the axle housing must also be installed, if necessary.
Knock
Knock, which can occur on all driving phases, has several causes including damaged teeth or gearset.
In most cases, one of the following conditions will occur:
1. A gear tooth damaged on the drive side is a common cause of the knock. This can usually be
corrected by grinding the damaged area.