Howdy! Loosen belt and check clutch pulley bearing for noise/wear/runout. They are not very expensive, and many of us have Yorks that are VERY old. It could be the belt is wearing out or has a bad spot on it. Could be more obvious when NOT under load. My belt has a very long straight run on one side and it vibrates pretty bad when under load at over 1500 rpm. I just need to add an idler pulley. JohnI have York OBA and when not engaged it has a vibration around 2,000 RPM. Belt tight and pulleys in line. Solid 8 bolt mount. Any ideas?