What Might Cause Rattling Noise While Accelerating and How to Fix It

If you hear a rattling noise coming from your car, that’s not good. It could mean a number of things that might be wrong, and most of them are pretty expensive to fix! Read on to see why these sounds might be coming from your car and what you might need to do to fix them. 

5 Possible Causes of the Rattling Noise When Accelerating

#1 Damaged heat shield

If you hear a rattling noise when idling and accelerating, it may be a damaged heat shield. Heat shields are pieces of metal that literally shield parts of your car from other — much hotter — parts of your car, like many parts in your exhaust system (like your manifold or catalytic converter). These thin pieces of metal are often close to the ground (unless you’re in a truck with a lifted suspension), which subjects them to road debris. 

Between that and the potential to rust, these are one of the more likely causes of rattling noise when accelerating. You can even climb underneath your car and check to see if you can find it. All you have to do is go underneath and shake the exhaust around to see if you hear any metal-on-metal clanking noises. Be sure to let your exhaust cool for at least one hour after driving before trying this! You can seriously burn your hand if you don’t let your car cool down first! Another good indicator it’s the heat shields is if you hear a somewhat loud clanking noise as you go over a speed bump. 

#2 Broken engine mounts

If your car engine makes a rattling noise when accelerating, particularly when you’re driving uphill, it may mean you have damaged engine mounts. Engine mounts are made of metal and rubber to keep your engine securely attached to the frame of the car. When the rubber becomes worn out, the mounts loosen and metal-to-metal contact happens, which can cause a rattling noise. 

#3 Issues with the valve train

The valve train controls which valves are open at what time in order to keep the correct air to fuel mixture in your motor. This is sometimes caused by using the incorrect octane fuel your car requires and causes a buildup of carbon inside the engine, making the parts stick. 

You must check your owner’s manual to see what octane of fuel you’re supposed to be using. A car like a BMW, Cadillac or Mustang will likely require a much higher octane than a Honda Accord, Honda Civic, or Toyota Camry. This can also be caused by having the wrong motor oil (or old, gunky motor oil) that’s not lubricating the valves correctly. If you hear a loud rattling noise coming from the engine when accelerating (some describe it like glass bottles clanking together), it may be a sticky valve train. 

#4 Transmission issues

Whether you have an automatic transmission or manual transmission, if you don’t have enough transmission fluid, it may cause a rattling sound when accelerating. Low transmission fluid means the metal gears within the transmission won’t be lubricated like they should be and may cause a rattling noise when accelerating. This is one particular issue that, if you ignore the problem, it will get much, much worse, and you’ll likely be replacing your transmission. 

#5 Damaged catalytic converter

The catalytic converter uses a honeycomb of thin, expensive metals to clean up some of the exhaust gasses that come out of the engine before releasing them into the atmosphere. 

Because the honeycomb structure is so thin, it’s easy for the metal to deteriorate over time and just sit in the converter. These little bits of metal rattle around inside the body of the converter. At idle, you might hear a low volume rattle. During acceleration, you will hear it get louder. An unfortunate fact about catalytic converters: they are commonly stolen because the precious metals inside them are worth a lot of money. The Toyota Prius is the most commonly targeted car for catalytic converter thefts, with the Nissan Altima and Mazda 6 ranking high on the list as well. 

How Severe Might the Consequences Be?

If your car makes a rattling noise when accelerating, clearly something’s up. How severe the consequences of ignoring this rattling and continuing to drive will depend on what’s wrong. If it’s a light rattle from broken pieces of metal inside the catalytic converter, it doesn’t really affect the car’s driveability. (Though it might negatively affect its ability to pass a smog test, which, if you fail a smog test, will prevent you from driving the car legally.) If the rattling is coming from the transmission, it’s likely that ignoring it and continuing to drive it will result in a pretty serious issue. And you don’t want to be in a moving car when your transmission goes out. 

The same can be said for a car where the rattles are coming from worn-out engine mounts. If it’s the engine mounts causing the rattles, it’s likely that one is causing the rattling sound, but the other three are similarly worn out. Hitting a big pothole hard enough is enough to cause some severe damage to the engine mounts, and in serious cases, could knock the engine off the car’s frame. 

How to Fix the Rattling Noise When Accelerating

The repair of the rattling will completely depend on what’s causing it. A damaged heat shield can be welded back into place, but it’s likely that if it’s damaged to the point of rattling, the metal is very thin. When the metal is too thin, it won’t do its job as a heat guard and may cause other damage to the parts that need shielding from the heat in the first place. A catalytic converter can be fixed, but only if it hasn’t begun rattling or has been discolored or smells of sulfur. 

If it is the cause of the rattling sound coming from your engine, it’s already too late to repair it and will need to be replaced. Fixing a motor mount will cost quite a bit of money because of the labor costs associated. You need an engine hoist to lift the engine off the mounts while you replace them, then they need to be swapped out, and the engine mounted back on the new mounts.
Fixing a transmission rattle will take some time because it will likely need to be removed and inspected to see what’s happening inside the transmission housing. It could be any number of parts inside the transmission, so it’s hard to say how exactly you would fix it — but we can say this: transmission repairs are expensive!

How Much Might The Repair Cost?

This all depends on the cause of the rattle and how bad it has gotten already. 

The cost of fixing damaged heat shields will be a couple of hundred bucks ($200-$300), and you may even get it done cheaper if it’s not too damaged. If not, you can get it welded back on for cheaper. But something like a damaged catalytic converter doesn’t even have to be fixed right away. The issue is that if the rattling is coming from your catalytic converter, it means it’s broken, and when it comes time to pass a SMOG test, you will need to replace it, or your car will fail the SMOG test. If you're concerned that your car won't pass smog due to issues with the catalytic converter or other related problems, it's essential to address these concerns promptly. Getting a catalytic converter replaced will cost you between $1000 and $2450. 

Replacing an engine mount will cost you at least between $220 and $570, depending on the make and model of the car. This will very quickly add up to an expensive repair, as most cars have 3 or 4 engine mounts each. If it’s your valve train, you’re looking at at least $900 to fix it and up to $1800, on average. Prices may be even higher depending on the make and model of your vehicle. 

Your transmission issues may be solved simply by putting some $30 transmission fluid to top it off if you’re lucky. But that’s if you’re lucky. Chances are if your transmission is already rattling, there’s some more serious damage than just low transmission fluid, and transmission repairs are notoriously expensive. If it’s really bad, you’ll need to replace your transmission, which runs from at least $1800 all the way up to $3400 on average. 

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