[klavant]

in my 06, i hear 2 knocks (2x) when i start the car and it is cold outside and the car hasn't run in several hours. The noise seems to come from the rear passenger side. I can't figure what is back there that whould make a noise like that upon starting, other than the fuel pump maybe? Has anyone else run into this?

 

[gar187er]

fuel pump.....nothing else would knock/make noise in the rear when turning on a vehicle

 

[Voodoodaddy]

Check your exhaust and rubber hangers. If the hangers are damaged or broken, the exhaust can contact the body, making noise, especially on start ups. The thermac doors in the dash have a tendency to make noise on the gen-I models on start up, but that noise is heard in the dash, unless it resonates differently in your car.

 

[adultswiivi]

I have an 06 and I get the same noise some times. I only hear it when sitting parked some where with the engine on. and its always been on normal running temps in summer and winter snow. It really sounds like someone walked up the the back side and knocked on the side of the nox, like KNock Knock.

Never figured out what it was.

 

[Voodoodaddy]

I heard the noise in my own car this last week, and never gave it much thought. It (mine) was 4 knocks, "Knock....kno, knock, knock".

I believe it's the fuel tank vent valve, which is part of the evap monitoring system, located in your fuel tank. There's a pressure sensor on the fuel pump and it measures pressure, then modulates the opening and closing of the of the vent valve.

It only does it after being started cold, (and only once after being started), or after a long period of sitting.


Here's the system's discription, and how it works:

EVAP System Operation
The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP canister vent solenoid valve to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP canister purge solenoid valve ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP canister vent solenoid valve OFF, open, fresh air will be drawn through the solenoid valve and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP canister purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

 

[RIT333]

Great info. This may save someone a trip to the dealer for no reason.

 

[Voodoodaddy]

This is funny.... Cause i never really noticed it before until now. I live 2 blocks from my kid's school, and it does it, almost religiously, as i leave the parking lot, after dropping her off. ;D

Like I said.....it's done it since day one and we've owned the car, (2005) since 2007 and it had only 14K on it.

 

[Voodoodaddy]

And on a more intersting note......... if you shut your car off, and sit in it for 15 minutes or more, with the engine off (I was on the phone.....) you'll hear 2 more "knocks". Likely the valve opening after pressure releases or equalizes.

Yeah..... I have that kind of time to kill. ;D