It could be the recirculation flap mechanism.
I had the 1/2 gear part that operates the recirculation flap break. It closed recirculation flap all the time, even when the knob was not set on it.
This gear is a half gear located under the dash, near the center hump on passenger side. You need to remove the cover & look for broken plastic.
I did not change it myself but the mechanic knew right where to go & as I watched he pulled out the part.
It looked fairly easy to check & replace.
08 CRD @ 163,000
OME HD Lift about 3"
AEV Wheels 5.75 BS
GSA 255/75/17 32.1" (no bolt cut/no pinch weld)
spare 265/70/17 stock location
Lockpic, 100,000 club