A rattling noise under the car when driving can be pretty embarrassing when pulling up to the traffic lights. Somehow cars have this ability to produce annoying rattles and bangs randomly. The good news is that some possible problems are easy to repair, so establishing the cause and applying the correct fix will eliminate that annoying rattle in no time.
The easy to fix cause of a rattle under the car when driving
Exhaust heat shield
Running the length of the exhaust tunnel and attached to the car’s underside above the exhaust is an aluminum heat shield; it comes in multiple sections connected with tiny screws or even tacks welded into place.
As the bottom is open to the elements, the screws get wet, rust, and the heat shield becomes loose. Once corrosion sets in, the heat shield becomes brittle, and you will find a large hole around the screw. As you drive, the metal heat shield can rattle under the car.
A rattling exhaust heat shield is the most common cause of a rattle. Almost every other car that goes into the garage with an under-the-car rattle is found to have a loose heatshield.
How to fix a rattling heat shield
Because the heat shield protects the underside of the car from getting too hot, it is best to keep the heat shield in place. Refit it with the old or new screw and a large penny washer. If you can’t find a penny washer big enough, you can cut open a coke, make a washer/section big enough to cover the hole and drill a hole for the screw.
Some mechanics will advise you to do away with the heat shield; however, you should be cautious. Certain exhaust parts, like the catalytic converter, can reach over 1000 degrees Fahrenheit. If the car’s underside becomes hot enough, the car’s internal carpet or debris could catch light.
The exhaust itself is suspended by rubber mountings attached to metal hangers. The rubbers become dry and break with age. Rubber mounts are used to allow the exhaust to flex when the engine moves under acceleration and gear changes. Broken rubber mounts enable the exhaust to touch the car’s underside, especially over bumps in the road.
How to fix a loose exhaust
Replace the rubber hangers with new ones available from all motor factors. Even if you can’t get like for like another with similar hole spacings will work. Sometimes new rubber hangers can be tricky to mount because they are so stiff. Use a small WD-40 on the holes, and then try pushing them onto the hangers. It will be much easier. If that doesn’t work, use a set of water pump pliers to force the rubber onto the hangers.
Engine dirt shield loose
Most new cars and some older ones will have a plastic tray clipped into the vehicle’s underside directly below the engine to stop dirt and debris from getting into the engine bay, where it could cause damage. The plastic tray can get damaged quite easily when driving at speed from debris in the road. You may have seen other cars driving along with the plastic shield flapping in the wind. It isn’t a metal rattle, but it can be pretty loud when driving.
How to fix to a broken engine shield
Just like the exhaust shield were possible, it should remain in place; it is there to protect against any other problems occurring; depending on the amount of plastic broken, you might need to drill the plastic dirt tray and affix it with cable ties. Make sure it is well supported. You can sometimes replace the shield. A new one will clip straight back into place, but usually, when the shield breaks, the plastic it mounts to breaks as well.
The not so easy fixes
We say these aren’t such easy fixes because they can’t necessarily be carried out at home without proper tools or replacing the offending items.
Above the exhaust is suspended by rubber hangers, but the metal hangers the rubbers fixed to rust and break with age. The exhaust can no longer be supported, so it rattles and bangs on the car’s underside as you drive and go over bumps. It may be possible to weld the bracket back on the exhaust or use some wire to re-fix it. However, replacing the exhaust may be the only option depending on the cause of its breaking.
Suspension problems typically cause a rattling under the car, but steering issues usually accompany them. A broken coil spring, worn ball joints, worn shock top mounts and anti-roll bar linkages are usually places for suspension rattle. Unfortunately, if you find an item damaged or excessive play(movement) in a ball joint, it will need replacing to solve the problem.
Failed catalytic converter
Inside a catalytic converter is a honeycomb of precious metals. Usually, through no fault of the catalytic converter itself, usually over-fueling or under-fueling, the catalyst fails and becomes loose, causing a horrible rattle as you drive. Giving the catalytic converter when cold with a tap with your hand, you can hear if the internals is loose. There isn’t much you can do apart from replacing it; if you carry on driving, parts of the catalyst will continue to break down, and you can ever have a clogged catalytic converter, or the internals are forced out through the exhaust, the emissions will all be wrong, and you may have the engine management light appear on the dashboard.
Is it safe to drive with a rattle under the car?
Depending on the cause of the problem, it may be safe to drive in. It will not cause immediate danger to yourself or further damage to the car, no matter how badly it rattles. But you need to know the cause of the problem before assuming you can drive.
An exhaust heatshield loose, for example, won’t cause any further issues unless it breaks away completely, which will be hazardous for other road users. However, a suspension problem like a broken coil spring is unsafe to drive; the car may randomly snatch you off to one side of the road if the broken spring gets caught in the spring pan.
A rattle under the car when driving can be alarming; however, the most common cause for rattling is exhaust issues like a loose heat shield. These are simple items to repair without needing specialist tools or replacing expensive parts. Simply re-fixing will solve the problem. The issue comes when it is a component of the suspension causing the rattle, in which case this is not always possible to replace at home without the correct knowledge and tools.