I'd been under the dash a dozen times tightening everything, rerouting wires, checking the speedo cable. Nothing. It sounded like the rivets holding the weather stripping in were buzzing.
Finally took my Boeing Engineer friend for a ride--it's nice to have an engineer on call, especially a past Chief Aerodynamicist for Boeing military aircraft! Anyway, he couldn't find it either but when I asked where he thought it was coming from, he circled the wiper and horn buttons. I reached over and touched the horn button an VOILA! No more noise.
It's incredible to me that this little all brass button vibrated enough to sound like the motor mounts were loose in the whole car! But that's what it was. I used some 1 and a half thousand feeler gauge stock and shimmed the button against the housing. No more buzz. Am I the only one to experience this? David