Vern,
I had an old lathe that made a clacking sound whenever the spindle was commanded to stop. I put a couple of shots of grease in a grease fitting there, and lo and behold, it also made no difference. So that's fairly similar, huh?
Is yours one of those without a counterweight? What is the Z axis servo motor load look like? Not that I'd know what it should be, but someone else may recognise if the % load seems abnormal.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)