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Question DetailsAsked on 8/4/2017

What does it mean when your car knocks? Its always from the left hand rear I have a 2005 Chevy Colbolt.

It happens consistently everyday and it knocks exactly three times every time it does happen.

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1 Answer


If this happens when the car is not moving - most likely even as soon as you turn the key on (even without starting the engine) as I suspect, and especially since it is always 3 clicks or knocks, it is almost certainly the gas tank evaporative filter solenoid clicking - several GM models including yours have a LOT of blog comments on this sounding like someone tapping on the trunk or rear panel with their knuckle. The solenoid goes through a reset and purging cycle - it is part of the air pollution equipment, located at the gas tank, and is a normal sound, even though in most makes/models of cars it is not noticeable to the driver but sometimes is commented on by rear seat passengers.

Other possible causes in some cars -

1) rear vent / AC damper cycling (again most likely only at startup) - flapping shut

2) if occurring ONLY when you hit a bump or pothole driving, could be the springs are bottoming out because your shocks are shot, or a suspension issue, or something loose in the trunk.

3) if during driving on smooth pavement or over small bumps (but not continuous sound) could be loose or broken suspension element, loose wheel, wheel bearings giving out, something loose in the trunk banging around

4) if intermittent while driving even on smooth road, could be a wheel or axle issue - worn/broken gear tooth, bearings going out, etc. If it occurs only when you change the load on the driveline - accelerating or decelerating, then most likely a bearing going out or worn splines on driveline, or gear mesh incorrect due to wear in driveshaft or axle.

5) again when you first turn key on (even without starting engine) electric fuel pump kicking in - though usually just a whirr with maybe one click, not three that I have ever heard

Answered 3 years ago by LCD

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