I have a 2004 Nissan Sentra that makes noise when fan is on (even when engine is off). This noise comes from behind the dashboard and sounds similar to s baseball card in a bicycle wheel. Noise goes away if I change the ventilation to recirculate from external air. Any Idea what this is
Probably leaves or something similar that have fallen down the fresh air inlet and are now touching the blower wheel.
Where do I look to fix this
You’d have to remove the blower to get at whatever is in there. When I get a leaf or something in the blower of my car I usually just wait until it clears itself, which it always does eventually.
Sometimes running the blower on high will clear it out.
I agree. I once had leaves in my Thunderbird vents and the heater fan resister caused them to put smoke into the car. Luckily it did not catch fire.