Q: My 2009 Volkswagen Jetta Sportwagen – with a five-speed manual transmission – makes a weird tapping noise. The noise has been there forever, yet the car runs great. I have had the engine looked at.

Engine knocking, or detonation, occurs when fuel in the cylinders ignites unevenly, causing metallic pinging sounds, reduced power, and potential engine damage. Key causes include low-octane fuel, ...