I've noticed that whenever I turn off the car, theres a lingering noise - almost like water running through pipes - that's the best way I can describe it. And it does not stop until I open the doors! After I open the door the noise stops.

Have you experienced this? If so, what is it?

 

It's probably your AC lines....the last several cars I've owned have done this. But on the safe side you should take a look and make sure nothing is leaking.....

 

Yes if you are running the a/c, after turning the HHR off the a/c is releasing pressure in the system