Engine noises: how to identify the source

An unusual engine noise can have many origins. Describing it (clicking, whistling, knocking, when cold or hot, at idle or under acceleration) helps the garage pinpoint the cause. Here are the most common leads by type of noise.

Clicks or knocks

Rhythmic metallic clicking at idle or under acceleration can come from injectors (especially diesel), valve clearance or, more seriously, timing or big-end issues. On diesels, injector clatter is often normal; excessive or irregular noise can indicate a faulty injector. A dull, regular knock can suggest big-end or piston trouble: have it checked quickly.

Whistles

A high-pitched whistle can come from the auxiliary belt (worn, slipping or faulty tensioner), an air leak (intake hose, gasket), or the turbo (leak or bearing). Whistle that increases with engine speed is often linked to turbo or belt. A cracked or slipping belt should be replaced to avoid failure (and sometimes loss of alternator, power steering, etc.).

Noises when cold or hot

Some noises disappear when hot (timing play, belt) or only appear when hot (expansion, leak). Noting when the noise appears (start-up, idle, acceleration, under load) and in what conditions (cold/hot, town/road) helps diagnosis.

What to do in practice

Do not ignore a new or worsening noise: a broken belt or disintegrating turbo can damage the engine. Describe the noise to the garage (audio recording possible) and state the conditions. A professional can listen to the engine and check belts, turbo and timing. My Auto Check-up helps you prepare your visit by listing symptoms for an initial cost estimate.