Q: Car is making a knocking noise when stopping. Other mechanics say it's motor. Some say it's the piston

Unfortunately, there is no way to tell without doing some more testing. The timing and small details about the noises you are hearing may make a huge difference in diagnosing the issue.

Lifters will make more of a "clicking" sound if they are not lubricated or are starting to wear down. These clicks will go away at higher speeds. If you are replacing one, replace all of them. The best way to inspect these is to open up the valves and take a look. This can be time consuming. One of the most common reasons for knock is due to improper combustion. New spark plugs can help tremendously if this is the case. The timing of the spark can also be an issue.

You may want to inspect the spark plugs and distributor as well. They might be the reason the combustion is off time. Engine knock can also be caused by fuel delivery issues, or poor fuel quality. Piston and engine knock will typically sound better as the engine warms up. A compression test can help give you insight to the condition of the pistons. This will take a special set up that plugs into your spark plug socket. If you would like to have this diagnosed, a certified technician from YourMechanic can come to your car’s location to inspect the noise and advise you on repairs.