Speedometer Cable Howl

[coldwar]

I just got back from a short highway jaunt with my '76 W115 (300D), and after reaching 70 MPH and staying there for a few minutes, I was startled by a very loud howling noise. I was on my way to a truck stop anyway to buy some engine oil, and was almost there, so didn't pull over immediately. As I slowed on the exit ramp however, the noise decreased in pitch and intensity and when I got below 40, the noise stopped completely. On the way back, it started up again when it reached 55 MPH. I set my cruise at 65 and noticed the speedo needle wavering rhythmically between 60 and 70. The cruise was working OK. Again, on exit ramp, the noise stopped below 40. The howl is very loud, and doesn't seem to come from the speedo directly, but more from the left of the dash. What do I need to fix this- new cable, new speedometer, lubrication or ?? (Attached is a photo of my dash)

Dave