I think you are describing two different noises, both of which are normal (if they turn out to be what I am about to suggest they are). Let me start by asking: Did you have your air conditioner running before you turned off the car? (I am refering to the one time the noise(s) did show up).
Well, if you did have the A/C on here is my reply:
When you say that it sounds like "air escaping in short bursts and diminishing as time elapses", this sounds to me like the freon running back into the low-pressure side of your A/C system. It sounds something like (Psssssss.... Ppssss..... Pppsss... Ppppss).
This noise / sound usually last a minute, no more than two minutes after the car is turned off. It is perfectly normal for the car to make this noise.
The second noise you described went something like "water falling on top of a hot stove". Well.... Once again, if you had your air conditioner running before turning the car off, it is normal for water to slowly leak out of the pan that is situated under the A/C evaporator. The evaporator, in simple terms, is a small radiator underneath your dashboard. The A/C coolant runs through its inside and air is cooled when it comes in contact with its fin. This produces "condensation". This condensation (water) is dumped outside the cabin after being collected by a drain pan underneath the evaporator. It is possible that the slow drip of water from this pan is falling onto the muffler or some other HOT surface underneath your car.
This is more than you asked for, buit I hope it helps.