The last few days commuting in my '81 240D manual tranny are driving me nuts. I get up to around 62-65 mph and begin to hear a strange soft rattle/buzz coming from it seems the passenger side dash. It isn't dependent on engine speed (it doesn't go away when I push in the clutch and the rpm's drop) but is definitely is related to speed of the car. After the car drops below 60 it disappears. It just started doing this a few days ago. The other clue is that it seems to not do it while it's cold. After I left the house this morning I got it up to highway speed and didn't hear the noise until several miles down the road.

My speedometer is about 3-5 miles off, it does like to jump at speeds under 20mph. Could this be the cable making noises?