I'm not sure if you are aware but there was some reported problems with a whistling sound inside the cab on the Twins, it was tracked down to the cabin filter door not seated properly. So if/when you change yur's pay attention to make sure it goes back together properly.
Here is a link that talks about it. Whistle
It's just the truck whistling while it works.
2015 Canyon SLT, Copper Red, 2WD, CC, SB, V6, Tow, blk leather, factory spray liner, safety pkg, generic roll up bed cover, Tekonsha P3 Brake Controller.