Usually there's vibration, but there can be many different causes for it - 1) the carriage may have some play along the ways (tighten the gibbs), 2) tool post stiffness, 3) tool bit too far out (shorten the reach to stiffen the cutter), 4) heavy cuts can make the cutting bit bounce (take lighter cuts), 5) feed too fast (slow the feed), 6) dull cutting bit, 7) bad geometry for cutting bit (bit grabbing and releasing), 8) Lack of lubrication or 9) difficult material to machine (some materials are just gummy and grabby like lead and copper). A lot of it you learn by trial and error.