On my last carve I heard a change in pitch of my machine and noticed a vibration in the flex shaft. The vibration is only on the Z truck end (I have a Carve Tight spindle) and goes from the Z truck up about 8 inches and then disappears. The flex drive has never been overheated. I inspected the core and spring and found nothing wrong. Both ends of the shaft are square and I did the twist test with less than 1/4 turn. I turned the spring and core around (end that was in spindle now is in the cut motor) with no help. I suspected the spindle bearings might be bad so ordered a new Z truck. I installed that and still have the vibration. After more research here found that rotating the top hat may help so tried that. The vibration was reduced a little but not much and after running for about one minute using the cut motor test found the spindle getting hot, the flex drive was not even warm yet. The first inch of the core was hot but I think that was just heat transfer from the spindle. Now I don't know if the new spindle is bad or could the shaft vibration cause the spindle to run hot. Guess trying a new flex shaft will be next unless someone has another idea.