Fisker

Just purchased my 2020 Denali HD with the 6.6L gas. Apx 350 miles on it now. Sitting in the drivers seat, at idle I hear a gurgling/hissing sound coming from behind the dash. To my ear, seems to be in the upper left area of the engine compartment in line with the tach display. AC unit and climate control fans are off. As I increase From 500rpm while in park, the sound decreases. When The tach reaches about 800rpm it sounds like a valve closes and the noise stops. The sound does not change when I turn the steering wheel. The sound is not noticeable with radio at low volume. My 2017 Denali 1500 never made this noise nor any other vehicle I’ve owned.