Engine Knocking When Warm – Causes and What to Do

Is your engine knocking when warm, but running quietly when it's cold? You start the car in the morning, everything sounds fine, you set off, and 15 minutes later there's a rattling noise you've never heard before. Your first thought is probably that it's nothing serious and it'll go away on its own. But this is one symptom I never like to dismiss at the workshop, because sometimes a harmless-sounding ticking noise is hiding a problem that could destroy your engine. And in this case, time really is money. The sooner you find out where the noise is coming from, the better your chances of avoiding a major repair.

How can I tell if my engine is knocking or just rattling?

First, I want you to pay attention to exactly what the noise sounds like. A fast, steady ticking from the top of the engine is one thing. A dull, metallic knocking from somewhere underneath is something quite different. If you press the accelerator and the knocking gets louder as the revs rise, we've got a good reason to investigate. Sometimes the oil warning light comes on as well, and that's one warning I really don't want you to ignore.

I often hear owners say, "There's nothing wrong with it when it's cold. It only starts rattling once it warms up." I understand why that feels reassuring when the car starts normally and drives fine, but it doesn't mean everything inside the engine is healthy. We need to find out where the noise is coming from, not simply shrug and say these engines all sound like that. Because there's a huge difference between an annoying rattle and a knock that's warning you about serious wear.

Why does an engine start knocking when warm?

The answer has a lot to do with engine oil. When the engine is cold, the oil is thicker. As it warms up, it becomes thinner, and the oil pressure can drop. If there's a worn component inside the engine that relies on a film of oil to run quietly, that's when you might start hearing the problem. Imagine two metal parts that need a constant layer of oil between them to stop them touching. When that oil film is no longer enough, the knocking begins.

That's why hearing "but it doesn't knock when it's cold" doesn't reassure me. Quite the opposite. It's an important clue when we're diagnosing the problem. The engine is telling us when something is going wrong, and our job is to find out why. Not wait until the knocking is so loud that people walking past can hear it too.

Which parts can cause an engine to knock when warm?

There are several possible causes, and they don't all mean an expensive engine repair. But! We can't tell them apart just because the engine rattles when it's hot. We need to work out exactly where the noise is coming from and how it changes with engine speed. Here's what we check.

• Hydraulic tappets. These are small components in the cylinder head that help the valves work properly. When something goes wrong with them, you'll often hear a distinctive ticking from the top of the engine. If you'd like to see what they look like, take a look at my article on the TDI cylinder head up close.
• Low oil pressure. The problem could be the oil itself, the oil pump or a blocked oil pickup strainer, which is the mesh filter the pump draws oil through. When the pressure is too low, the components inside the engine don't get the lubrication they need.
• Crankshaft or connecting rod bearings. Now we're talking about something more serious. These bearings allow the main moving parts of the engine to run smoothly. When they wear out, you can get a deep, metallic knocking noise. I've shown you more about them in my article on main bearings and the engine block.
• Timing chain and tensioner. If the chain isn't kept tight enough, it can start rattling. That's why we check this possibility too, rather than immediately blaming the bearings.
• A heat shield or loose clamp underneath the car. Yes, sometimes it's really that simple. A metal part starts vibrating as it heats up and makes a noise that sounds like an engine problem. That's why we inspect things first and start taking them apart afterwards.

Will thicker oil stop the engine knocking?

You'll often see people in online groups saying, "Just put thicker oil in it and it'll quieten down." And yes, it might actually become quieter. But that doesn't mean you've fixed anything. If the bearings are worn or the oil pressure is low, thicker oil might simply hide the symptom. It's like turning up the music so you can't hear something rattling under the bonnet. The problem is still there.

I don't recommend experimenting with different oils until you know what's causing the noise. For cars driven in Bulgaria, I recommend changing the engine oil and all filters – oil, fuel, air and cabin – every 10,000 km or once a year, whichever comes first. Even if you're using LongLife oil. This helps look after your engine, but it's no substitute for a proper repair when there's already a mechanical problem.

How do we find out exactly where the knocking comes from?

At Das Auto Service, we don't start replacing parts based on guesswork. First, we need to establish whether there's a lubrication problem, an issue with a moving component or simply something vibrating outside the engine. There are several checks that help us find the cause without making you pay for repairs you don't need.

1. We check the oil level and measure the oil pressure with a pressure gauge. We don't guess whether the oil pump is working properly. We measure the actual pressure.
2. We listen to the engine with a mechanic's stethoscope. This tool helps us pinpoint where the knocking is coming from – the cylinder head, the bottom of the engine or somewhere else.
3. If necessary, we carry out a compression test and inspect the inside of the engine with an endoscope. That's a small camera that lets us see what's going on inside. I've explained the process in detail in how we check an engine's health.

Only after these checks can we talk about a specific repair. Don't accept a verdict that your engine is finished just because someone heard a rattle and decided they knew what was causing it. A proper diagnosis should give us answers, not more guesses.

When should I stop driving immediately?

I know nobody wants to be stranded at the side of the road, especially when the car is still running. But if there's any suspicion of a bearing problem or low oil pressure, every extra minute the engine runs could make things worse. It's much better to arrange recovery to the workshop than to risk destroying the entire engine. I'm telling you this because, at the end of the day, you're the one who'll have to pay for the repair.

Tip from Kris Mladenov: If your engine only rattles once it's warm, record the noise on your phone while you can hear it clearly. Sometimes, the moment you arrive at the workshop, the noise disappears. That recording will help us understand what you heard and where to start looking.

I hope you've found this article useful! If you have, I'd really appreciate you sharing it with a friend who's wondering why their engine starts knocking when it's warm. My mission is simple – when I know something useful, I want more people to know it too, so they can look after their cars properly. Because getting something checked in time can save you a lot of headaches.

The solution in our shop

Genuine Longlife III FE 0W30 oil 5L

Genuine LongLife III oil is a suitable choice when it meets the oil specification stated in your car's owner's manual. Using the correct oil is important for lubrication, but it can't repair components that are already worn.

View in shop

Read more

• Diagnostics
• Engine overhauls
• Engine Rebuild: Main Bearings and Engine Block
• Engine Compression Test: How We Check Engine Health
• Inside the TDI Cylinder Head: Camshafts and Lifters

Frequently asked questions

Why does my engine knock when it warms up?

As the engine oil warms up, it becomes thinner and the oil pressure can drop. If there are worn bearings, faulty hydraulic tappets or another component with a problem, that's when the knocking might start.

How can I tell hydraulic tappet noise from worn bearings?

Hydraulic tappets usually make a ticking noise from the top of the engine, while worn bearings can cause a deeper knocking from the bottom. But I wouldn't diagnose an engine just by listening to it. We need to check the oil pressure and pinpoint where the noise is coming from.

Can I keep driving if my engine is knocking?

If the knocking is deep and getting louder, or the oil warning light is on, stop immediately and switch off the engine. Don't risk driving any further until you've found out what's causing it.