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Discussion Starter #1
Does anybody know what might cause a knocking coming from the back of the vehicle. It really does sound as if somebody was knocking on the trunk lid from the inside. The first time it happened when I just started the engine, just three knocks in quick succession. I shut it down and restarted, same thing. Then nothing for about two weeks and one morning it happened again when I started her up. This time I was able to determine it’s definitely originating at the rear of the vehicle. I immediately thought, "fuel pump" although there has so far not been any effect on the engine's preformance. This afternoon when I got home and shut down, I remained in the car for about five minutes reading something from work when I heard it again, without the electrical on. Although it didn’t sound as loud as before, it was four distinctive raps coming from the back end. It’s only happened about half a dozen times since late February, once while driving and rounding a turn (if that could mean anything). I plan on telling my dealer when I take it in for an oil change but I’d like to have some kind of idea what it is when I do. Other then this the car runs like a dream. Any ideas out there?
 

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OMG I thought I was the only one hearing that! I don't know what it could be, but it really does sound like somebody knocking. Only thing I can think of is something or other heating up/cooling down, but I honestly don't know.
 

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I am pretty sure the fuel pump is in the back near the trunk.
 

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dude i hear it too!!!! i always freaked out but now i try to jump out of my car after i shut it off to hear where its coming from. it always happens about 20 seconds after my car turns off.
 

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Discussion Starter #6
At quick glance the first thing that came to my attention is our's are all '07s, anyway it's just an observation. I'm leaning towards the fuel pump, but the fact that I still heard it when I had the ignition off and no electrical power going to the pump makes me rethink.
 

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06 Cobalt

Mine does it too. I have an 06 Cobalt LT. I dont think it is the sound system because it used to do it when I had regular speakers in the doors and now it still does it when I upgraded everything on the inside of my car. It is always three knocks about 20 to 30 seconds after I turn the car off.
 

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well it seems to be a common thing I think this was posted up before i think it had to do with something in the exhaust? and something calibrating itself? I have an '05... i searched and couldn't find the thread.
 

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Discussion Starter #12 (Edited)
Any of you guys have the premium sound package?????
I don't have XM, or a CD changer but I have the MP3 and the other audio options minus the steering wheel controls, if that could tell you anything.

IronSilk6 added 14 Minutes and 40 Seconds later...

well it seems to be a common thing I think this was posted up before i think it had to do with something in the exhaust? and something calibrating itself? I have an '05... i searched and couldn't find the thread.
I'm not 100% sure how long after I turned the ignition off today that it happened. I was paying more attention to what I was reading so it might have coincided with the computer resetting itself and maybe it does have something to do with the exhaust or the emissions. If it is the computer doing something with these things or even the fuel pump it might explain why it happens right after startup as well. But I'm not so sure if that explains it happening so randomly while I'm driving, where as everybody else’s is more consistent.
Anyway I feel a little bit better seeing it's a bit more common then I thought and everybody’s balt seems to be in great shape. I’ll let the dealer know, they’ve probably dealt with this before and at least it’ll get noted in their computer as a pre existing problem in case it causes the whole thing to crap out later on.
 

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I hear it too and I have a 05!! Would the battery and such have a hand on it? I am sure its fuel or pump or brakes...any SS have this same worry? (if its a drum or disc brake thing?)
 

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Its totally normal, its the EVAP system going through a cleaning cycle. There was a TSB out to the dealerships to not replace it because it was not malfunctioning
 

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The noise and or tapping you are hearing is the evap canister and the can's vent valve which is located behind the fuel tank just above the rear cross member. The system periodically tests itself by pressurizing and then releasing back into the atmosphere.
 

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i have the same noise every once in a while in my 07. i asked the dealer about it and they said it is normal. has something to do with evap relieving pressure or something. i forget what exactly they told me
 

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Discussion Starter #17
Excellent, thanks a lot everyone this puts my mind at ease.
 

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now that you guys mentioned it, i heard it this morning. scared the crap outta me cause it was like dawnish and still kinda dark :lol:
 
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