Digging up an old thread. My 2016 Yukon Denali with only 39,000 miles is having the same issue, and I also noticed the same thing as Keith Berry. It *seems* like the sound stops/goes away whenever I shut off the climate control.
Keith, or anyone else, have any idea on why it seems shutting climate control off would cause this? I'd chalk it up to chance, but seems like I'm not the only one that noticed this.