The worn taper on the pulley. It has a 10 degree angle, so I set the cross slide to 10 degrees and claeaned the surface. My boring bar was almost too short, and I had a bit of chatter on the feed in. I had to grip the pulley on the oil seal surface at the back as the pulley is not centered if you grip the outside. That made the distance from the tool holder longer and allowed for the vibration. Nothing to be done about that except buying a sturdier longer boring bar, so I live with this.