I think I finally found the source of the buzzing
in my dash. It's actually not in the dash, just sounded
like it was coming from the center of the dash. It's
the rear-view mirror. I was able to recreate the noise
by revving the engine until the buzzing started. Then
moved my head around the dash until I got close to the
mirror. Tapped on the mirror and sounded very close
to the annoying buzz that's been bugging me for months.

Not sure how to fix the problem. Doesn't look like
the mirror comes apart. Maybe some lead strips like
you get with overhead fans to balance them?

Grant