So I noticed a couple weeks ago that there was a high pitch whistle when I turn my fan on, regardless of whether the A/C is on. The whistle gets louder when the fan is on higher. I also noticed that if I crack one of my windows open, the whistle instantly stops. Does anyone have any ideas of what is causing this noise?
Thanks,
Reed

 

I also noticed that when the air is switched to recirculating, the whistle stops. Any ideas?
Thanks,
Reed

 

I guess I stumped the chumps. My only thought is something to do with an air filter or something stuck in the vents or something.
Reed

 

So I realized I didn't include my car details. I've got a 2006 Civic EX with about 135k on it.
Thanks,
Reed

 

I finally figured it out. I thought it might be something stuck in the cabin air filter so I checked it only to find the filter wasn't fully seated. I pushed it all the way in and boom, problem solved.
Reed