When AC is cut down low, there is a loud fan noise on the passenger side under the dash. This just started happening about two weeks ago.
that would be the blower fan for heat and a/c. depending on the sound it is possible that debris got in their like leaf’s etc. if it is a groaning or squeaking sound it might me the blower motor causing it. but being a newer vehicle, I would doubt it.
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I expect WW is correct above that debris from the air inlets has worked it’s way into the fan area. Your shop will have to figure out a way to access that area and do a visual inspection. Hopefully it will be a simple procedure & they’ll be able to just vacuum the debris out…
Could be something else of course, but a visual inspection of the blower area seems like it is probably necessary in any event.
The debris problem can often be avoided by the owner vacuuming the hvac air inlet area under the windshield periodically. Especially remove any pine needles. I keep a small shop vac on the ready just for that purpose & vacuum that area on my vehicles whenever I change the engine oil and filter. It’s easy to do on my Corolla, a little more complicated on my truck.
Your vehicle has a cabin air filter.
So debris in the fan wheel is highly unlikely.
The blower motor may have a bad bushing causing the noise at low speed.
Tester
Your fan is bad and close to shutting off.
MB designed a climate system that allowed the bottom of the fan to be too close to the accumulated water and “schmutz” with the eventual result that the fan’s bottom bearing is going to pot.
Solution:
- Buy a replacement motor and wheel, $50 - $150 parts cost.
2 Remove the motor, soak the heck out of the bottom bearing while rotating the fan with “3 in One”, “PB Blaster”, etc. being reasonably careful to keep the oil out of the electrics.
My personal opinion is that the result is roughly the same, lasts the same duration and is realtively inexpensive assuming you follow the Utube videos, dont mind buying a cheap set of Torx and playing a bit of Twister, which at 65+ is getting harder to justify so you might just want to take it to the dealer and paying $200/hour.
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Do you mean water from the AC system? In that case good idea for OP’s shop to check & clean the AC drain vent at the same time.
It’s water from the intake vents, hot and cold, with all the crud that manages to get by the inital filterf.
Because the pan is so close to the pan (with water and crud), eventually the lower fan bearing get’s compromised.
And if MB wants “real user” experience to correct their engineering errors, I’m more that happy to oblige. 
.
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