What is this?
I recently started hearing a soft squeaking noise from behind the dash (center/Rt) of my '86 300E, w/90xxx miles. Sounds like something heavy is loose in there, makes the sound mostly when going slower over pavement ripples, or when decelerating. an oscillating sound, like "squeak-squeak-sqeak", usually lasts less than a second at a time, but is still annoying considering how quiet the car is otherwise.

I don't know, perhaps the speedometer cable based on your description. A few drops of light machine oil down end of cable might do the trick, if, in fact, it is the speedometer cable.
Ralph

I'll wager it's the air sampler motor. There's a small electric motor driven fan that samples the air for the Tempmatic climate control.

If that's what's come loose (the sound is definitely that of something that's begun rocking back/forth or up/down), is there a relatively easy way to get to that motor to secure it?