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the_yellow_dart

knock knock knock knock knock

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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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unless friends sneak into your trunk.... i'm just gonna guess it's a loose muffler bracket...?

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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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Forgot to mention - it always happens when I'm stopped.

Can't see how a loose muffler bracket could cause exactly five knocks, evenly spaced, every time.

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Oh, and it's not a power antenna or CD changer. Only thing back there besides the standard components in every car is the Pioneer subwoofer that came with the car. The sound seems to happen whether the radio is on or off, it doesn't matter.

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Forgot to mention - it always happens when I'm stopped.

Can't see how a loose muffler bracket could cause exactly five knocks, evenly spaced, every time.

Does it happen right after you stop or after you've been stopped a ten seconds or so?

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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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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.

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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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The EVAP purge valve is up front, but the vent valve and canister it is attached to should be in the rear, very close to the tank.

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I'm with 'bee on this one.

Chris

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

Yeah. That noise is usually coming from their transmission solenoids.

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Yeah. That noise is usually coming from their transmission solenoids.

I always assumed that was the transmission countdown, and eventually it was going to blow up.

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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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I've been reading about a little quirk of most Toyotas. They run away with the driver and crash at full throttle. Cute, eh?

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Purge canister solinoid at the tank, not sure if thats the proper GM name for it or not.

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