Has anyone successfully fixed the whistle issue that some of us get on our trucks? when my truck is sitting in the driveway with the heat going, I get a very pronounced whistle from the vents(?) that gets louder as you crank the heat up each notch. No whistle from the vents when the heat is off or when driving with the heat off, but I’ve been unable to find where the whistle is originating from. I’ve tried opening and closing the vents, removing them all together, recirculating on/off, changing it from windshield to all floor and everything in between...nothing changes. It still whistles no matter what I do. the shop dropped the blower motor down and couldn’t find any obstructions or anything so I’m at a loss. any ideas?? Thanks in advance!