Most probably the spindle bearings, headstock and tail of the spindle. The Southbends I used would have to have attention paid to them on occassion. We had shims between the bronze bearings on the headstock and when the bearing became worn, we would remove a shim and use hi-spot to see how the spindle was rubbing on the spindle and scrape accordingly.
We would run our tool bits upside down and reverse spindle rotation to put the cutting forces DOWN onto the headstock casting bearing to stop tool chatter until we could fix the looseness of the bearing to spindle.