buzzing speedometer
 

i was driving my 83 300td last night and the speedometer started buzzing very loudly. as soon as i would go over 35 mph the needle would bounce down and the speedometer would buzz about 3 times louder than any of the warning buzzers. it would be louder and higher pitch with more speed and wouldn't stop buzzing until the car was almost at a complete stop.
has anyone had this problem before?
i have to drive 12 hours today and i will not be able to handle that at 70 mph for 12 hours.