I have a 2013 Mazda 3 GSX 2.0L. 70,000 KM (43,500 Miles). Popped the hood after a long afternoon stuck in heavy traffic - like over 2 hours just to make sure coolant levels were all good, purely hypervigilance.

Noticed that after about 10-15 seconds after shutting off the engine there was a little 'Shhh' sound. No whirring, grinding or other odd sounds like bubbling or hissing (yes, I've been down that rabbit hole this afternoon). More like the engine was saying "Shhh" to get my attention. The sound lasts about 1 second. Nothing before or after that. I have come to the conclusion that this is quite normal. Can anyone confirm?

Many thanks!

 

Its common for solenoids and electric motors to stay active for a few minutes after the key is turned off. Throttle body is common.

 

Because of miles on engine and you said "shhhhh" as if air was passing......As an experiment temporarily disconnect the breather line/hose going into the Throttle Body boot between the throttle body and air box. Start the engine let idle and shut off. If no sound re-connect and try again. If the sound returns you need to check your PCV valve and all accompanying hoses for restriction ,function and blockages. The sound after shut down may be that the crankcase is pressurizing and the PCV system and breather may not be properly controlling that pressure correctly.