Accessory drive belt noise may be described as a thumping noise heard at, or just above idle speed, as a result of belt pilling. In a single-belt accessory drive, belt pilling occurs when small particles of belt material separate from the belt grooves, and form small balls or pills in the belt grooves. While some belt pilling is normal, excessive pilling can cause a thump or knocking as the pills pass over various pulleys. This causes the belt to repeatedly jump off of and slap onto the pulley.

The thump or knocking noise easily misdiagnoses as an engine noise. In order to diagnose belt pilling, adhere to the following procedure:

1. Remove the drive belt.
2. Run the engine for no more than 30 to 40 seconds, in order to confirm that the engine noise disappears.
3. Inspect the power steering pulley for proper alignment.
4. Inspect all other pulleys for excess paint, debris or pills in the grooves.
5. Clean the pulleys with a wire brush, as necessary.