Hissing noise after turning off ignition

[jupiter2step]

I just wanted to check whether it is normal to hear a short hissing noise - as though air is being released from somewhere? It happens every time I turn off the ignition and lasts only a couple of seconds. I don't have the airmatic suspension but was wondering whether it has something to do with the air con stabilising itself or the SBC?

 

[BlackC55]

Where does it come from?

 

[jupiter2step]

Not sure... can't track it down. Somewhere under the bonnet. As I say, it only last a few seconds then it's gone, so by the time I pop it open I've missed the opportunity to track it!

 

[Supacool1]

Could be the turbo spooling down...?

 

[Frankie]

Blowers?

 

[DRBC43AMG]

A nest of snakes perhaps?

 

[macy]

Hi,
my C220 cdi makes exactly the same sound on switching off the engine. Lasts about three seconds then stops, I think the sound is coming from the area of the heater but so far have not been able to pin-point the cause.
Theres a hissing sid in there somewhere.