Hey guys, Im experiencing a similar issue on my 17' Sierra but it's not a whistle.
I hear a soft whine in idle, when accelerating in 1st and 2nd gear, and when I slow down to come to a stop. I don't really hear it after 2nd gear but it definitely gets louder when I hit the gas in low gears.
I bought an AFM disabler thinking that was possibly the issue but am still having the same noise.
I had a 5.3 in my 2013 Silverado and never heard this whine so I guess that's why I notice it so much with this truck.
Transmission fluid is good and full.
I've read a ton of forums and it sounds like it could be the fuel pump, water pump, or "normal" as some mechanics have told people.
Any advice or help would be great to ease my mind or point me in the right direction.
Thanks