knocking noise

im trying to figure out what it is.. it is a knocking noise, like three really fast knocks.. sounds like it is coming from the fuel filler area (drivers side)

this is for a 02 impala 3.4L

anyone else hear this noise? what is it?

edit* it is three very fast knocks.. doesnt do it all of the time

usually does it within the first 5 minutes of startup/driving when everything is mostly cool

literally it sounds like someone is outside knocking on my car trying to get my attention

Sitting still, driving, over bumps? Still not enough info!

Sitting still, driving, over bumps? Still not enough info!

am i missing something?

please read my first post

"usually does it within the first 5 minutes of startup/driving when everything is mostly cool"

it doesnt matter if it is sitting still, driving, or going over bumps

it only does it one time though, once you hear it (and you dont hear it every time) you wolnt hear it again for usually the rest of the day

edit* sorry my post isnt ment to be mean, iv been in a bad mood recently, dont take anything personally...

Within five minutes of startup, and it does it while moving or standing still?

One guess is that the fuel tank expanding after being parked from engine/exhaust heating warming the fuel and creating higher vapor pressure in the sealed tank. When the engine is started cold, the EVP system draws a slight vacuum on the fuel tank to test its integrity, and the tank may be collapsing slightly from vacuum. I believe the tanks on those years are polyethylene, and might make such a sound under vacuum, but so would a metal tank. The vacuum would tend to remain until the car is shut off for a while.

One way to test this would be to loosen the fuel cap on startup and see if the noise occurs in the usual amount of time. However, this might also set a trouble code due to the tank not being able to hold a vacuum. If you have an easy means of clearing trouble codes (like a scanner) you could test this. If you DO have a scanner, you should also be able to watch the fuel tank vacuum as the engine starts ans see if there is a point where the noise occurs.

It's just a guess.

am i missing something?

please read my first post

"usually does it within the first 5 minutes of startup/driving when everything is mostly cool"

it doesnt matter if it is sitting still, driving, or going over bumps

it only does it one time though, once you hear it (and you dont hear it every time) you wolnt hear it again for usually the rest of the day

edit* sorry my post isnt ment to be mean, iv been in a bad mood recently, dont take anything personally...

it is possible that the cat exhaust is expanding and this would rapidly heat up during the first five minutes as unburned gases are greatest....

has it been a long time since plugs/wires checked/replaced....? a large plug gap would cause extra unburned gases to heat up the cat extra hot..

thanks for the replies

it very may well be the vacuum thing you describe, but it doesnt happen all of the time.. it is actually quite rare, but seems to happen more when it is cooler outside.

i have two gm cars, and they both do it, and the sound seems to be coming from the same exact place.

that is a great idea, but the sound is the exact same every time, even in both cars, im not sure if the tank expanding/contracting would be able to produce the same sound every time.. i would think it would be different.. not trying to say you are wrong, because i really have no idea

second, i doubt it is the cat, because well the noise isnt coming from anywhere near where the cat is, in my camaro, which makes the noise also, the cat is up front basically under the glovebox, and im assuming that is where the cat is in the impala, or somewhere near there.. this noise is comming from the back drivers side quarter panel area