Noise from under the bonnet that shouldn’t be ignored

Noise from under the bonnet that shouldn’t be ignored


They indicate a failure in the accessory drive, and in the valvetrain. Usually, a whistling sound occurs due to a stretched belt or a broken tensioner. Humming sound is a symptom of a broken bearing. It is very dangerous to ignore these malfunctions.
For example, a jammed bearing of the cooling pump or the alternator may cause belt breakage, which leads to complete inoperability of the ancillary equipment.
It is even worse if unusual sounds are heard from the timing belt: its breakage leads to valve deformation and results in engine overhaul.


This sound usually appears when the variable valve timing mechanism fails. Sometimes, it can be heard when the camshaft position sensor is broken. Failure of these elements results in engine malfunctions, thus reducing its power.
Sometimes, a rattling sound may be caused by a stretched timing chain. Like with the belt, this failure should be fixed immediately.


It indicates worn engine mounts or failure of a spark plug.


It is one of the most common symptoms of engine malfunction. It indicates many failures: valve damage, worn camshaft bearings, cylinder and piston failures. The nature of the knocking sound may be used for pinpointing the defect.

  • Thumping sound that appears after the engine warms up and turns into twang when the gas pedal is pressed. It indicates critical wear of camshaft and connecting rod bearings.
  • Ticking clearly heard from a cold engine immediately after it is started. It may completely disappear after warming up. It usually indicates failure of the valves.
  • Uneven dull knocking in the oil pan that changes frequency together with the engine’s RPM indicates destruction of crankshaft bearings.
  • Knocking clearly heard immediately after the engine is started and disappearing in several seconds may indicate an insufficient or an excessive level of motor oil and clogging of the oil filter.
  • A sharp metallic sound, that is suddenly heard from a relatively new engine and gets more intense as the load increases, may indicate engine detonation caused by using low octane fuel.


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