Disassembled, found hardened grease with metal shavings which were teeth from large drive gear. Ordered new gear set and grease, Cleaned gear box of all old grease, inspected all bushings and remaining gears for damage. Assembled mixer dry to check contact pattern of all gears using grease as pattern marker. NOTE: PTO gear slightly larger than original for deeper gear contact. Mesh of gears was good so took unit apart and reassembled with plenty of recommended grease. Mixer runs better and quieter than when new.