Milling a Diameter problem......backlash?

Have a Haas VF-0 1992 and noticed today that when milling a diameter I'm getting the slightest step in the OD where the X axis changes direction. After noticing it in the first few pieces you can actually hear the machine changing directions in the X axis (a slight thud can be heard), but the Y Axis there is no sound at all. I was interpolating at 100 IPM and it was noticeable, when slowing it down to 50 IPM it was very slight but the resulting step in the OD was the same. I measured the part and it is round within .0002", but over the step it was larger by .001" When milling an OD I get a step at the 9:00 o'clock position, and when milling an ID I get the step at the 3:00 o'clock position. Both OD and ID are being climb milled. I would think there would be a step at both 9:00 and 3:00 on the OD and ID but that is NOT the case. Doesn't make sense to me, so hopefully some one will know what the solution is.

Best Regards,
Russ

I had the locking nut for the ballscrew loosen up, same problem. I just had to lock it back up and has been good since.

I am just curious which nut you are referring to? The ball nut, AC bearing nut on the screw, or the floating end nut?