MY 240 produces a screeching noise at about 65 mph, though sometimes I need to go faster. The sound comes from behind the steering wheel and can be produced at lower speeds (like 60 mph) if a large vehicle passes me in the oposite direction. So it's clear air flow does something. I've read this is the odometer cable. How hard is this to fix? The noise isn't bad when I'm in town but when I need to travel the noise for hours on end is driving me crazy.
Warren

I've had something like this on the 83, but only when there was a strong crosswind from the right and I was at highway speeds. High-pitched, maddening buzz. It was loose window trim. Check all the trim around the glass by tapping lightly with a fingertip until you find a loose piece. Also check the radio aerial if you have the one on the A-pillar. If it can't be tightened by any other way, try using a bit of silicone or Goop to glue it and check if that changes the sound. Then on to a more permanent fix.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

Does the noise sound like the noise we used to make when we were kids and we would take a leaf or a dollar bill or a sheet of paper, pull it tight and blow on the edge of it?

There is some sort of flapper valve in the air inlet system that brings air in from the grate in front of your windshield to your heater. When the air flow reverses direction through that system (like when you meet a truck) it can make that noise. I've never heard as a continuous sound it under normal driving, so yours air ducts may have some trash in them or something else wierd going on.

A '91 240 does not have a speedometer cable. Instead, the speedo is driven electronically from a sending unit on the rear axle.

It's possible that what you are hearing is actually a drive belt slipping. They get old and glazed, and do not grip the pulleys as well as they should. Usually, the belt(s) slip at a certain engine speed. Run the car up through the lower gears and if you hear the "screech" note the engine speed on the tach. The belt should "screech" at this speed in all gears.

Another possibility is the center carrier bearing on the driveshaft. But this is more of a "roar" than a "screech."

Forgot to ask: can you describe the sound in more detail? (Hence the header of the post above this one).

You do not have a speedo cable with the electronic speedo.