My heat is either all on or off. I installed a known good CCU and the heat is the same. Thanks to a timely write up by Chris W from the gasser forum, on how to R&R the fan, I tried placing a square of paper against the sensor grille. Nothing. Earlier this week and in the past, I could here a motor running while waiting for the glow plugs to do their thing, so I was pretty sure the fan was working.
Yesterday and today, no motor sound. So, when I find the fan hiding under the vent, the rubber hose is disconnected and when I powered the fan, it ran silently. I removed it and split the case. I put a couple drops of 3 in 1 oil on the shaft. Figured it couldn't hurt. I spliced on some extra hose to make it easier to put back. When I tested the motor again before installing it, it made the same noise I had been hearing, The impeller was lightly touching the housing. I was able to slide the impeller a little to get the needed clearance.
It is silent again and will hold up the little square of paper.
I haven't driven the car yet. I would really like to start running the heater with the temp wheel, instead of the Off and Economy button.

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1987 300D 146K Original #14 Head
One eye on the road, the other on the temp. gauge

Gone But Not Forgotten
1983 380 SEL Long gone
1983 300SD Wish I hadn't traded it in on a 90' Corvette
1989 300SE My all time favorite
1995 E420 Went like a bullet