Howling from my Dashboard

[Codifex Maximus]

I was driving home yesterday in my 300D when a weird looking pickup passed me; I heard a howling noise and thought, "He needs to get that thing fixed." He kept on going whereupon I realized that it was *my* car making the noise.

I panicked! Sounded like a WW2 hand crank siren. I checked my oil pressure, brakes, engine had good power, no steering problems. Pulled over and as I slowed down, the sound diminished. Stopped the car and the sound went away.

Got back into the road and accelerated to speed and all seemed well. After a while, the sound came back. I noticed that the dashboard was humming and I could deaden the noise by pushing on the instrument cluster.

I've come to the conclusion that my speedometer is failing. It acts erratically when the sound starts.

Any thoughts? Possible remediations? Is it possible to oil the cable or speedometer working parts or just replace it?