by Immediately after starting, many cars can have a very loud engine noise. Even if the car is brand new, the engine hums significantly during a cold start. Many drivers are concerned about this, although in reality, it is not a malfunction. 
The fact is that modern fuel-injected engines have a specific algorithm for warming up to operating temperature. The electronic control unit gives a command to increase engine speed.
This is done intentionally to: – ensure that engine parts are more effectively lubricated with more viscous cold oil; – make the engine run more steadily; – warm the catalyst to its working temperature, as it only purifies exhaust after reaching 350 degrees Celsius. This is why the engine is loud during warm-up in cold weather. 
As soon as the engine reaches working temperature, it becomes quieter. It should also be noted that the source of noise can be increased clearances between engine parts. Cold metal contracts, and the clearances, for example, between aluminum pistons and cast iron cylinders become larger.
As the engine temperature rises, the metal expands, and the clearances become smaller. The impact loads on the piston decrease and the engine becomes quieter. However, in cars with over 100,000 km of mileage, the cause of increased noise during warm-up can be clogged oil channels in the hydraulic lifters. The oil pump simply cannot effectively pump more viscous cold oil into them. As soon as the oil heats up, the knocking disappears.
It should be noted that according to reviews and statistics, Japanese car engines are the loudest when cold. Specifically: Mazda, Nissan, Subaru, and others. “Hum” is also prevalent in large-displacement engines, resembling tractor or truck engines upon starting. But after a short time, operation stabilizes, and the noise subsides. Therefore, the simple conclusion is that engine noise at start-up doesn’t always signal potential malfunctions.
It’s just the nature of engine operation, nothing more. However, if the hum does not subside after warm-up, that is a cause for concern. Regardless of the car brand and engine type, remember that the key to long-lasting and trouble-free engine operation is timely maintenance and Hado motor oil.
