When I turn my motor off, I hear a slow but quite prominent 'psssshhhhhhhhh' sound, like something had strong vacuum on it, then when the motor was turned off, you hear the vacuum leak down again.

I believe it is the weird multi-orb vacuum reservoir thing in the centre console? That looks like a few light bulb sized globes connected together?

I'm not sure that's where it's coming from, but from that general area.