Sounds like either you cable broke or you guage cluster is FUBAR'd where the speedo cable broke. My '96's speedo did this when I got it in March. Made noises too. About a week after I got it the thing goes nuts on my morning drive to work and then the needle goes dead. Turns out the cable was all corroded and dragging, making it vibrate and making the needle twitch like that, and that wore out the guts of the speedo.
New speedo cable later (local Chevy dealer, $47ish) and new cluster (TVT over on Zuwharrie.com - price varies), and it's all fixed.
The tach has nothing to do with the speedo and odo so it still working means nothing in this case.