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knock knock knock knock knock

the_yellow_dart

By the_yellow_dart
in Tech Section

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the_yellow_dart Newbie

the_yellow_dart

Posted
Posted

This is something I don't think I've ever posted about before, even though it's happened since I bought my Cobalt.

Occasionally (I'd say once a week), I hear what has become a very familiar noise from somewhere in the back end of my car. It is exactly five quick 'knock' or 'tap' sounds, about 1/2 a second apart. It literally sounds like somebody knocking on the trunklid - the first couple of times I looked around my car to see who was knocking. It's not loud, as I notice that passengers usually don't pick up on it. I've never noticed any problem stemming from this sound occuring.

It's not a problem, but I'm just curious - What could this be?

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SAmadei Newbie

SAmadei

Posted
Posted

This is something I don't think I've ever posted about before, even though it's happened since I bought my Cobalt.

Occasionally (I'd say once a week), I hear what has become a very familiar noise from somewhere in the back end of my car. It is exactly five quick 'knock' or 'tap' sounds, about 1/2 a second apart. It literally sounds like somebody knocking on the trunklid - the first couple of times I looked around my car to see who was knocking. It's not loud, as I notice that passengers usually don't pick up on it. I've never noticed any problem stemming from this sound occuring.

It's not a problem, but I'm just curious - What could this be?

Its the poltergeist of the classic Chevys melted down to build your Cobalt. All new cars do it.

Or did you forget and leave your hostage in the trunk?

Seriously, does it sound like its coming from the trunk or under the trunk? Does it happen while your driving or sitting still?

Do you have power antenna back there? Mine was pretty noisy when it died and my current one has been knocking at me, telling me its going to die soon.

I doubt you have a CD changer back there. They can be noisy.

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SAmadei Newbie

SAmadei

Posted
Posted

fuel pump?

After I've been stopped ten seconds or so.

Yeah, fuel pump starts to be my next thought. Is this problem correlate to your fuel level?

I'm also wondering if, under the correct fuel level and braking conditions if the pump is causing tank is getting a pressure differential enough to cause some pressure to escape or enter the gas cap.

I also wonder if it could be a suspension bushing binding up. Squirting them with lithium grease can sometimes make that so away.

I had a weird knock/light clunk in the rear of the Bonneville... and I checked the suspension, fuel, trunk hinges, everything... then, suddenly, the passenger side rear control arm broke! As it turned out, there was a crack in the arm, and it was knock was the crack flexing. When it finally let go, the ball joint and the tip of the control arm fell onto the bottom of the wheel, luckily my mother wasn't driving it fast.

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SAmadei Newbie

SAmadei

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Posted

Its most likely the vent valve for the EVAP system. My VUE has always thumped three times just like that anytime its below about 60°F outside. IIRC, the EVAP canister and valve are the same on the VUE, ION, and Cobalt/G5 so it makes sense.

But thats on the front. His noise is in the rear.

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  • 2 weeks later...
ocnblu Newbie

ocnblu

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Fightingbee is correct, according to my old employer (local Chevy dealer). When I took mine in to them for the steering column replacement, I commented on the same exact sound, 5 raps in quick succession. It is that vent valve. Disconcerting in that I've never had a veh that did this, but now that I know it's normal, it falls under the radar. Every car has its quirks.

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  • 4 weeks later...
Drew Dowdell Grand Master

Drew Dowdell

Posted
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Fightingbee is correct, according to my old employer (local Chevy dealer). When I took mine in to them for the steering column replacement, I commented on the same exact sound, 5 raps in quick succession. It is that vent valve. Disconcerting in that I've never had a veh that did this, but now that I know it's normal, it falls under the radar. Every car has its quirks.

like the buzz/tap sound that every Chrysler FWD automatic makes when decelerating to a stop?

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Intrepidation

Intrepidation

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If that's the case mine has been counting down for the last 199,400 miles :P

It's a quirk but it's normal, and yes its the solenoids, they do it on every upshift and downshift but you only year it when they're coming to a stop because the engine rpms are low and youc an hear it over the engine.

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