jshoota

I have a 2020 3.0 Duramax diesel. Similar sound at same "certain rpm" fan speed, mainly slower. Whistling noise started at 15,000. I checked to make sure all vents are open. Troubleshooted through all vent settings (defrost seemed to make it louder). Only setting that makes it go away is the recirculating mode. Hate to keep it on recirc mode because it takes it off Auto mode.