Climate control pushbutton unit rattles/clatters like jiffy pop when cabin temp gets close to what's set on the temp dial. I can move it to a more extreme temp and it will stop until the cabin temp gets close to that setting, then it will clatter again. Sounds like a relay that can decide whether to turn on or off, so it just flutters between on/off. Anyone know what can cause this? Thanks.

With no info on your car its hard to troubleshoot.
I just had the same thing happen to my 82 300SD. Pull your control unit and look at the circuit board.
Do a search on re-soldering the control unit.

In my case the relays had broken away from the board. The temp wheel had a bunch of crud on it as well.
As you describe it would make a loud buzzing chattering noise.

I replaced mine with a new control unit I had on hand.
No further noise and the auto actually works now.

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86 300SDL 387,000? Motor committed suicide
81 300SD 214,000 "new" 132,000 motor
83 300SD 212,000 parts car
83 300SD 147,000

91 F700 5.9 cummins 5spd eaton 298,000
66 AMC rambler American 2dr auto 108,000
95 Chevy 3/4 ton auto 160,000
03 Toyota 4runner 180,000 wifes

The hysteresis circuitry in the unit isn't functioning, or its getting such noisy inputs that its being completely confused.

I concur.. swap it out, or at least try resoldering it, and see what changes. That's kind of the sledge hammer approach to diagnostics, but its a good first place -- if the problem isn't a loose connection to the temp sensor or a broken temp sensor (which don't seem likely), then its probably a loose connection or out of spec component inside the ACC itself that's affecting the hysteresis circuit.

Here's a shameless plug to my blog writeup from when I resoldered my ACC -- its all the way at the bottom.

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