may just be dirty and running it for 5 minutes or so at full speed will clear it out, but first check that the carbon brushes have plenty of life left in them. The other possibility is that someone has put oil in the lubrication ports and the motor will need to be opened and cleaned to remove it. Once clean, use original Singer motor lube (not the stuff they sell now) or Petroleum Jelly in the grease ports. You can spend big $$ on Nova Montgomery's or the Featherweight Shoppe's new motor lubricant, but it isn't necessary as Singer recommended PJ when they quit making the original motor lubricant.