Lately, I have been hearing a faint rattling sound from my car’s engine. It is most obvious when I am driving up a slope, even if I am not accelerating hard. It sounds like a moving part has come loose. What could be wrong?

A moving part which has come loose will result in rattling most of the time while driving, not just in the specific situation you mention.

The symptom you describe sounds like a condition called pre-ignition, sometimes known as “pinging”, which is an apt description of the sound. It is also commonly referred to as “knocking”.

While it sounds like mechanical noise, knocking is actually a result of severe pressure pulses in the combustion chamber arising from uncontrolled combustion. If allowed to persist, the engine could end up with failing bearings, burnt valves or even a crack in the cylinder head.

Knocking could be caused by different conditions, the most common of which is a low-octane fuel. Never use any fuel with an octane number that is lower than recommended by the vehicle manufacturer.

The other common causes of knocking are high temperatures and a lean fuel mixture in the combustion chamber. Check your car’s temperature gauge. If it shows a higher-than-normal temperature, have the vehicle checked and rectified immediately. Over-heating can result in serious damage to the engine.

A lean fuel mixture is not likely with most fuel-injected engines as it is fixed and cannot be adjusted. In the event of an induction air leak, which will definitely cause the mixture to be lean, the Engine-Check warning will appear.

It is with older cars where fuelling is via one or more carburettors when you could end up with a lean mixture and consequently knocking under load.