Strange sound when engine switched off

[Bootstrap]

Had my 2000 2.7 Base since Easter and loving being its new owner.
Going strong after a MAF and throttle body clean thanks to the combined advice from the knowledgeable members of this great forum.
Recently, however I've noticed a rather embarrassing sound being produced when the key is turned off and the engine stops. It lasts for around 1 second or so and sounds a bit like air is escaping from somewhere and making a brief noise not unlike a child's squeeze toy (for want of a better description).
It sounds like it's coming from the front of the car and I suspect the right side for no particular reason.
Any suggestions regarding a way I might locate the source of my car's apparent indigestion?
TIA.

[itsnotanova]

I guess you're not getting any codes? I suggest taking the front right wheel and liner off. Have someone run the car and turn it off while you listen for the noise. The charcoal canister is located over in that area.

[Frodo]

Mine does the same thing, if we're talking about the same noise. Over time I've kinda gotten to thinkin' it sounds pretty cool

We absolutely may be talking about different noises. Mine sounds a bit like a mini jet engine winding down, lasts maybe a couple secs..

[Bootstrap]

Thanks, Woody. I think you may have something there ...
The fuel cap was replaced recently with a genuine item and it has occurred to me that the sound has only been happening since then.
Maybe vapour pressure in the tank is leaking from somewhere?
I'll check it out this weekend.
Thanks for your suggestions.

[Bootstrap]

Quick update: before switching the engine off after a warm up run, I removed the fuel cap and the idle speed immediately increased but the sound did not occur at the moment of switch off. Repeated the process with the fuel cap installed and the sound returned.
Close inspection revealed that the large circumference of the genuine cap acts on a small plastic tab located in the side of the fuel filler opening that I suspect must open a valve of some type thereby releasing (or directing) fuel vapour to the appropriate location. It would appear that this valve is faulty?
Shortly after, I obtained the following codes: P1128, P1130 and P0440.
Would members have any idea about where one might look to fix this problem?
Cheers.

[Gelbster]

brilliant diagnostics !

[Paul]

https://youtu.be/zulI06TPBVc

[Bootstrap]

Thanks for the link Paul, however this elusive sound still persists even after replacement of the filler vent valve.
I've noticed the sound when driving and it seems to be occurring at regular intervals of a few seconds on and then repeats maybe 30 seconds later.
It only happens when the car is hot and naturally can not be heard when the car is stationary making it difficult to precisely locate.
I'm thinking that this may be related to the vent circuit associated with the plastic tab that is engaged when the fuel cap is fully installed.
This sound was not apparent when a non genuine fuel cap was fitted to the car.
It has only been audible since a genuine cap was recently installed by a repairer.
Is there another location or component that may be responsible for this unusual problem?
TIA

[Bootstrap]

For the information of members, the noise was traced to a faulty non return valve adjacent to the charcoal canister. A similar Audi unit did the trick at a total cost of $4.00.

[Starter986]

Originally Posted by Bootstrap

For the information of members, the noise was traced to a faulty non return valve adjacent to the charcoal canister. A similar Audi unit did the trick at a total cost of $4.00.

Awesome! Great detective work... and $4 later!

[NewArt]

Originally Posted by Bootstrap

For the information of members, the noise was traced to a faulty non return valve adjacent to the charcoal canister. A similar Audi unit did the trick at a total cost of $4.00.

And thanks for updating!