if it only does it once, it's normal. If it REVs......like up and down, up and down....then it's very likely a vacuum leak. Get out your helms manual (if you don't have one, go get a haynes at least down at the local auto parts store for all of $12 - best investment you'll make for $12).....and go through the tests that it describes.

It may also be your IAC valve, which there is a test for that in the Haynes/Helms as well. if that's the case, get a new one from a junk yard intake manifold. Also check for any unplugged vacuum hoses (I know it's obvious, but if you've been doing work under the hood, you may have forgotten one or something).

- Nathan