A squealing, chirping, or slapping noise from a drive belt. Could be the fan belt, alternator belt, or compressor drive belt (if belt-driven). Common causes: too loose, worn/glazed, misaligned pulleys, or contaminated with oil.

What you'll see

High-pitched squealing on startup that may go away as the engine warms up. Or continuous chirping/squealing under load. Slapping sound if the belt is very loose. The noise comes from the belt area, not from inside the engine or compressor. A slipping fan belt causes overheating because the fan and water pump slow down. A slipping alternator belt causes the battery to discharge because the alternator cannot charge.
Before you assume this is the problem

If the noise is a growl or rumble rather than a squeal, it is more likely bearing-related. See: Worn Air-End Bearings, Engine Mechanical Noise.

See all causes of unusual noise / vibration →

How to diagnose

  1. Identify which belt is noisy

    With the engine running, safely observe the belt area. Apply a small amount of water (spray bottle) to each belt while running. If the noise stops momentarily when water hits a belt, that is the noisy one. Water temporarily increases friction on a slipping belt.
    Result: Noise stops with water = identified the slipping belt.

  2. Check belt tension and condition (engine off)

    Push the belt in the middle of its longest span. Deflection should be 10-15mm. More = too loose. Check for cracks, glazing (shiny smooth surface), fraying, or missing chunks. A glazed belt slips even at correct tension. Check pulleys for alignment -- a misaligned pulley wears the belt unevenly and causes noise.
    Result: Loose, worn, or glazed = tension or replace.

How to fix it

  1. Tension or replace the belt

    Adjust tension using the alternator pivot or belt tensioner. Replace belts that are cracked, glazed, frayed, or stretched beyond adjustment. A new belt will stretch slightly -- re-check tension after 8 hours of running. Always carry a spare fan belt on site.

Common mistakes

Over-tightening belts. An overtight belt puts excessive side load on the water pump, alternator, and fan bearings, shortening their life. The belt itself also wears faster. The correct tension allows slight deflection -- not guitar-string tight. Also: oil on a belt causes slipping and rapid deterioration. If a belt keeps getting oily, find and fix the oil leak.

Parts & tools

Replacement belt (correct size). Wrenches for tensioner adjustment. Spray bottle with water for diagnosis.

Review safety precautions before starting →

Safety

Keep hands and clothing away from belts and pulleys while the engine is running. Rotating belts can catch fingers, jewelry, or loose clothing.

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