Weird fog-horn-like noise from right-rear after shut-down

[DPDISXR4Ti]

This is a new one. Yesterday after an ~hour long drive with the car parked in the garage I noticed a weird noise coming from the right rear. It was a faint, steady tone that lasted maybe 2 seconds, stopped for 5 seconds, and then repeated. It continued for at least 5 minutes. I was going to disconnect the battery to see if that would kill it, but by the time I got my 10mm wrench to disconnect the ground cable it had stopped.

My siren is already removed, so it's not that.

[Deerhurst]

My car does that too. Havnt figured it out yet. Hasn't affected the battery. I do have the "siren".

[Flyboy02]

Not an expert but that right rear is where low pressure fuel pump is. The system could be cavitating slightly (air in fuel pump line). Those pumps are under back seats on the right hand side. Could even be the gas tank cap. Again, not an expert.

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[Deerhurst]

Fuel cap makes sense. It's definitely right rear on my car. Sounds like it's from under the bumper behind the right rear wheel. That's not far from the fuel cap.

[shoemaker617]

its most likely a clogged fuel filter, some times it takes out the fuel pump...very common issue

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[Deerhurst]

its most likely a clogged fuel filter, some times it takes out the fuel pump...very common issue

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OP said it only happens when the car is parked. Shouldn't have the fuel pump running when the car is off and parked.

Mine also hums when the car is parked and engine not running. My fuel filter has maybe 5k miles on it. My fuel pump never makes a noise.

[Cybersombosis]

If it’s just a faint sound coming from the gas tank area, I’m thinking it is just the cap allowing air to enter the tank. The sound is most likely there with the car running but it’s so faint you can’t hear it over the engine and exhaust. There is a one way check valve in the cap which allows air to enter as the fuel is used up kind of like a diving mask. I have the same sound when I stop and park the car. Well that’s my theory anyways. I guess you could test the theory by taking the cap off and see if the noise goes away. You may get a CEL if you do this as an FYI.

[Kevin C]

My guess is something with the leak detection system of the evap control. This has happened to others.

The leak detection pump is located underneath the drivers side rear wheel well liner. It provides several different functions. When you turn the car off the LDP opens up and serves as a vent to prevent pressure build up in the fuel system. Any pressure build up in the tank gets forced through the charcoal canister and out through the air filter on the LDP discharge hose. It could be that the diaphragm in the LDP is making a noise. However, the noise would be coming from the drivers side rear and not the passenger side rear.

https://www.audizine.com/forum/showt...om-back-of-car

https://www.audizine.com/forum/showt...Detection-pump

[kwilson]

I had a B5 that made this sound when parked on a hot day. I never figured it out, but it didn't affect performance at all. Sort of a rhythmic electric noise? I think I narrowed it down to the leak detection system. Searching at the time revealed that some do it, but no real answers on what it was. I think it stopped when I unscrewed the fuel cap, so it was probably some pressure issue.

[A4SoftWalker]

Definitely related to the evap system. The evap canister is located above the right rear wheel well in the fender by the fuel filler cap. It has charcoal in it and if the fuel tank is frequently "topped up" the charcoal can become soaked in gasoline. Also the N80 valve under the hood is plugged electrically into the engine harness right next to the engine air filter and physically connects the engine air intake to the evap canister. It actually allows gas tank fumes to be sucked into the intake while the engine is running.

If the noise heard by the OP was fuel filter related then it will occur when the engine was running as a honking-scraping noise (Kinda like a goose or turkey) not when the engine has been switched off.

So the problem could stem from pressure being released because of:

1) Charcoal canister possibly damaged from overfilling the fuel tank or old age
2) N80 evap valve malfunction or
3) Hoses/lines connecting the system together are damaged or blocked.
4) Leak detection pump malfunction. (Also located with the evap canister under the right rear fender).

Not sure but there might be a code stored on the ECU if the N80 valve is faulty not sure if it will trip an MIL light. My experience here is over 12years ago. Someone might be able to chime in if it is not mentioned in the linked threads below.

My guess is something with the leak detection system of the evap control. This has happened to others.

https://www.audizine.com/forum/showt...om-back-of-car

https://www.audizine.com/forum/showt...Detection-pump

[Deerhurst]

My car does a little hum/honk/buzz thing from the rear sometimes after shut off. It's never thrown a code for it. Sometimes it won't do it. Sometimes it'll do it for hours. Sometimes it'll do it a couple times and be done. Sometimes it'll do it every day when I get home from work. Sometimes it won't do it for weeks.

[seanf86]

my 2002 does this, I just figured it was the leak detection pump or something doing its thing.

[xXxREBELOxXx]

When did you last have the fuel filter replaced? My ‘02 had the same noise and location intermittently as well.
Changed the fuel filter and it was gone completely.
If it is the fuel pump, the longer you run a clogged filter, the more wear you’ll put on the pump.

Fuel filter is something I’d change regardless for nothing more than piece of mind.


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[DPDISXR4Ti]

its most likely a clogged fuel filter, some times it takes out the fuel pump...very common issue

Fuel filter was just replaced about 5000 miles ago, so that seems unlikely.

[DPDISXR4Ti]

Definitely related to the evap system. The evap canister is located above the right rear wheel well in the fender by the fuel filler cap. It has charcoal in it and if the fuel tank is frequently "topped up" the charcoal can become soaked in gasoline. Also the N80 valve under the hood is plugged electrically into the engine harness right next to the engine air filter and physically connects the engine air intake to the evap canister. It actually allows gas tank fumes to be sucked into the intake while the engine is running.

If the noise heard by the OP was fuel filter related then it will occur when the engine was running as a honking-scraping noise (Kinda like a goose or turkey) not when the engine has been switched off.

So the problem could stem from pressure being released because of:

1) Charcoal canister possibly damaged from overfilling the fuel tank or old age
2) N80 evap valve malfunction or
3) Hoses/lines connecting the system together are damaged or blocked.
4) Leak detection pump malfunction. (Also located with the evap canister under the right rear fender).

Not sure but there might be a code stored on the ECU if the N80 valve is faulty not sure if it will trip an MIL light. My experience here is over 12years ago. Someone might be able to chime in if it is not mentioned in the linked threads below.

Thanks for the thorough answer. I'm going with this until I prove or disprove it. As a possible diagnostic step, couldn't I quickly disconnect the N80 valve when the symptom is occurring? If it stops, it confirms that it's something in the evap system.

[old guy]

Thanks for the thorough answer. I'm going with this until I prove or disprove it. As a possible diagnostic step, couldn't I quickly disconnect the N80 valve when the symptom is occurring? If it stops, it confirms that it's something in the evap system.

Just loosen the gas cap and see if it changes.

[DPDISXR4Ti]

Just loosen the gas cap and see if it changes.

Yes, good idea. I thought of that too after my earlier post.