Strange sound when engine switched off

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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.

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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.