Concerned On A Noise

[Hamza Type R]

My mate has ctr 52 plate 80k on clock drives perfect no eml on
and when he switch the engine off it makes a Gas type of noise for a second and stops
any ideas what that could be?

 

[Loxy]

Aftermarket intake?

 

[Hamza Type R]

Aftermarket intake?

Yes,not sure what make,it has a filter that looks like a mushroom,green colour
Its the HKS induction kit

 

[Loxy]

It will be pressures/vacuums from the breathers etc equalling out when you turn off the engine and nothing to worry about.

You hear them a lot more with open cone intakes, that's why I asked. The GruppeM is known for doing it too.