2000 Sienna Is Hissing When Shut Off? Options

[Julie]

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!