strange post-shutdown noise w/ a/c use

[auditude at cox.net]

Hi List,

I've noticed a new noise from my 200q20v after I shut it down when I've been using the a/c.  It sounds like a faint bottle rocket sort of a sound, like a missile in the distance changing pitch.

It's coming from under the hood, where I can also hear some faint hissing as if pressure is escaping from one place to another as things settle down.  I don't know if they are related or if the hissing indicates a problem.  The a/c seems to work fine, knock on zebrano.

Initially thought the whistle noise might be the radiator fan coming to a stop, but I don't think it is.  I'm going to check into it more somehow, but I was wondering if this rings a bell with anyone here.  And/or, is there anything in particular I can check?

Thanks,

Ken