We tested two different LTZs ... the first with sunroof, the second similarly equipped, but with the cab mounted clearance lights. Both were 2015s. We took the clearance light equipped out on the interstate and turned off the radio and heater fan to listen to the sound quality of the ride as there had been a background noise of whistling air ....it was as if a widow was down or a door not shut / sealed.
When we returned to the dealership and told them of the noise, they were at a loss to explain the noise ...other than the possibility of the marker lights causing the turbulence. The general manager is also a pilot and tuned in to vibrations or extraneous / new noises while underway; he understood my concern. We bought the truck and have experienced the noise intermittently. This may not be the same whistle as you are describing, but ... (GM- please send me a bonus reward - power folding mirrors, etc) ... we discovered the source of the whistle last weekend. The ventilated seats cause the whistle right next to your ear ... but make the whistle sound like it is coming from the roof or rear of the cabin !!! YMMV