It's likely the buzzer has an intermittent short inside. Not common, but it can happen (Hey, it's 21 years old). Your cheapest option is to find one from a pick-n-pull yard and swap it with your current one.

I'm not 100% sure where it is on the SD (The kick panel is the panel directly above your knees in the driving position), but you should be able to track it down by listening for it. It's a self contained silver can.