Exhaust rattle: causes

If your car has started to rattle from underneath, the cause is usually one of three things: a heat shield that has worked loose, a worn mounting or bracket, or something that has broken apart inside the exhaust itself. Some of these are minor and quick to put right, while others can lead to a failed MOT or a section of exhaust coming loose.

Working out which one you have comes down to when the noise happens and where it seems to come from. Here is how to narrow it down safely, what a garage does about it and when it is time to stop driving.

Why is my exhaust rattling when I start the car or idle?

A rattle at idle or when the engine is revving gently often points to a loose heat shield. These thin metal plates sit over the exhaust, particularly around the catalytic converter, to protect the floor of the car from heat. Corrosion eats away at the fixings and the shield starts to vibrate against the pipe.

A rattle from a cold start that fades as the engine warms can be a sign of a damaged catalytic converter, where the internal honeycomb has broken up. Shaking the exhaust gently when cool and listening for loose pieces inside is something a garage can do quickly on a ramp.

What causes an exhaust rattle over bumps or when pulling away?

Noise that appears over uneven roads or as you set off often means a hanger, rubber mounting or clamp is worn or broken. The exhaust is meant to hang slightly free so engine movement does not crack it, but when a rubber hanger perishes the pipe can knock against the underbody or the axle.

Corrosion at a joint can also let sections move more than they should. Road salt, short journeys that let moisture sit in the system and age all play a part in how fast this happens.

• Perished or split rubber hangers
• Rusted brackets or broken welds
• Loose or corroded clamps at joints
• Exhaust touching the underbody or suspension
• Heat shields with rusted fixings

What can I check safely before going to a garage?

Wait until the exhaust is completely cold. Hot exhausts cause serious burns, so never reach underneath after a drive. Then look for anything hanging low, obvious rust holes, black soot marks around joints and any part that moves freely when pushed gently by hand.

Never crawl under a car supported only by a jack. If you cannot see properly without going underneath, it is safer to have a garage put it on a ramp. Make a note of when the sound is loudest, because that helps with diagnosis.

How does a garage fix a rattling exhaust?

Once the car is raised, a technician can check every hanger, joint and shield and identify the source. A loose heat shield can often be secured with a suitable clamp or fixing. Worn hangers are replaced, and a corroded section may need repairing or replacing, depending on its condition and how the system is made for your vehicle.

What affects cost is the part needed, whether the system is in sections or one piece, how seized the fixings are and how long the work takes. We cannot give a repair promise without inspection, but we can explain what we find. If a catalytic converter has broken up inside, the answer is different again, and it depends on the vehicle.

When should I stop driving with a rattling exhaust?

Stop and get help if the exhaust is dragging on the road, hanging loose, or if you smell fumes inside the car. Exhaust gases contain carbon monoxide, which is dangerous in an enclosed space. Open windows and do not carry on if you feel drowsy or unwell.

A rattle can also become a failed MOT if the exhaust is insecure or likely to fall off. Automaster is a garage at Longcombe Cross near Totnes, and we can look the car over and talk through what is needed.