Hi there! I have a 2000 Sienna with 150K miles on it. It started hissing every so often a few months ago when the engine is shut off. It's pretty loud, and sounds like a tire losing air quickly. It's coming from under the passenger side sliding door (looks like a muffler, but isn't?)---is that the catalytic converter? It's leaking some water, about the size of a quarter and usually stops hissing within a few minutes of turning off the car. It seems to be running fine, but it's doing this hissing thing more and more. Thanks for an input!

1.
check to be sure you are not loosing coolant. insure you do not have a coolant leak, if so you will have much larger problem than a hissing noise.

2.
it could be water from A/C drain that is draining onto a exhaust component in that general location.
after you stop the van water will drain out of the discharge hose for a while. this water could be falling onto a exhaust component.

3.
guess you will have to crawl under car with flashlight and just look and listen.

let us know what you find, when you locate the problem.