Diagnosis
The chain is riding up on the teeth of the drive sprocket. This is due to a loose chain or a worn sprocket.
How to Fix It
Tighten the drive chain. Locate the chain tension adjustment bolt near the motor. Loosen the lock nut. Turn the adjustment bolt to take up slack. Leave about 1/4 inch of up-and-down play in the chain midway between units. Tighten lock nut.
If That Didn't Work
Inspect the drive sprocket teeth for wear. If they are hooked or sharp, replace the sprocket. This requires removing the chain and gear case cover.
Heads Up
A severely loose chain can derail and cause major damage. Address this immediately. Wear gloves when handling the chain; the edges are sharp.