When the Speedometer internal bearings wear and get dried out, the shaft can walk around the perimeter of the bearing instead of simply turn inside it. This is called "spooling", causes vibration, and makes a nasty racket that could be interpreted as a buzzer.
I had one that made noise in the morning when it was cold, but not on the way home when it was warmer.
I believe I finally lubed the cable and the cable socket in the instrument while it was upside down. Just two or three drops of speedometer cable lube. It hasn't done it again since. But, I'm not sure the lube did it or if it just got tired of whining.
Cheers,