We just had new ballscrews installed on our knee mill and are finding a
slight "vibration" or chatter while manually indexing the screw on the
x-axis. You can feel it occurring around the same position i.e. between 0
and 90 degrees rotation of the screw.

Their doesn't appear to be any interference issues between the screw and the
frame.

Any suggestions on trouble shooting this one?

The installer is being dodgy on addressing this issue.