Carlo, I see a couple of things. Your work was not supported by the tailstock so I'm surprised that it didn't chatter even more, that is a lot of stick out. Normally about 3x the diameter of the work is the max. This would indicate that your tool is working just fine.
When threading, as the tool goes deeper, the cut is wider, so more prone to chatter. Run another piece, and support the end with the tailstock center and I'll bet the chatter goes away.
In all honesty I had figured as much being unsupported. And the tool height wasn't right and and and. I just jumped into it and started playing. When I had made my scratch pass I had it kind of supported then I decided to
Move the tail stock because it was interfering with my movements. It's just odd I'm so used to hearing or feeling chatter. This just cut a nice curly chip with no noise whatsoever.
Regards-Carlo