Buick Park Ave Cruise Control Noise

I've got a 1999 Park Ave that all of a sudden has developed a loud "thunk" when engaging the cruise control or tapping the brake to take it off. Curise is working normal.
I imagine it's a relay, but frankly have never heard such a noise from a relay --- it's not a click. It's quite loud and annoying. Any thoughts before I take it into the dealer? Car only has 23K on it but it is out of warranty of course.
Thanks
Dan
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