Q: Engine noisy even after turning ignition off

Hello, It looks like you are experiencing two different issues here with your Mazda 3. First, the shaky steering wheel at higher speeds is due to the the vehicle’s tires being out of balance. Taking your Mazda to the nearest tire shop for balancing is the quickest way to get this concern taken care of.

Now, from my experience, the symptoms your Mazda 3 is exhibiting with noise is something called dieseling. Dieseling occurs when the fuel inside the cylinder is being ignited without spark. This is associated with high compression, heat and fuel entering the combustion chamber. This interaction is very rare in modern fuel injected vehicles.

It is also possible that your cooling fans are continuing to run after you shut off your engine. A way to test this is to take the keys out of the ignition and listen for something running under the hood. Open the hood and look toward the passenger side of the engine where the serpentine belt is located. Verify if it is moving. If it is, then your engine is indeed continuing to run without a key and if you see the electric cooling fans by the radiator moving, this can be normal.

Sometimes these fans will continue to run to cool down the cooling system as well as a/c system (if you’ve been running the a/c) This would be more common hotter days. If however these are not the culprits behind the noise you are experiencing with your Mazda 3, then I feel it is best to enlist the help of a certified mechanic to diagnose exactly what is causing the engine noise and diagnose shaky steering wheel.