It sounds like the serp. belt has come partway apart with the rotation (not across the belt) and is "slapping" against one of the pulleys.
We have a 97 also, and ours did the same thing after I changed the Lower Intake gaskets ( have you changed yours yet? Keep an eye on them). Scared the crap out of me. Couldn't see it with the belt on, but was very apparent with the belt off.
Changed the belt, and the problem went away. It twisted the cam in half three weeks later though.
