Several factors can cause this problem:

• Wheels Are Misaligned – To adjust them, refer to the “Adjust Wheel Position” section in your user manual.
• Drive Belts Are Worn – Replace them.
• Play in a Wheel – Tighten and lubricate the wheel bearing. To do this, refer to the “Tighten and Lubricate Wheel Bearings” section in your user manual.