Mercedes: flapping to buzzing behind middle dash, speedometer cable, squirrel cage

Question
After about five minutes of driving a flap to buzzing sound (this varies on my speed),somewhere in the middle to right side of dash.Also doing about 70mph on the freeway it starts making a intermitant whipping sound like wore out speedometer cable.

Answer
Steve,
Unless this is a very early model Mercedes,I would suspect that the speedo uses an electrical connection. If it were a mechanical chain, I do not think would wait five minutes before making noise.
You may be hearing a bit of insulation or other debris  that has been entrained in to the fan squirrel cage. Or the fresh air intake seal may be flapping against the box. Because all the duct work is under the dash or contained in the heater core box I am not sure you would be able to do anything without removing the dash.
Rob