Whistling noise when shut off

[steelcity]

If anyone can help, I'd sure appreciate it. 2016 2500 6.0 gas. Sometimes, (not always) when after shutting off my truck, it gives off a high pitched whistling noise. Sounds like a tea kettle boiling. Not constant, off and on.  It starts about 10 or 15 minutes after shut off. Not always, just occasionally. Coming from under hood but I can't pinpoint it. Dealer says it's some electrical items shutting down. I'm sceptical on that as it doesn't do it every time. Sounds like the truck is sending morse code in a boiling tea kettle sound, lol. Anyone else heard this noise? 

Thanks

Gary D.

[steelcity]

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[steelcity]

Well, I'm going to answer my own question, lol.  Dealer called and said GM just put out a bulletin and it states that the noise is from the internal transmission solenoids being commanded to minimum line pressure by the TCM. Noise is considered normal.  Naturally.  Just about every GM vehicle is affected from 2006 to 2019.  They gave me a copy of the bulletin.   Document ID : 5246136  #PIP4021D: High pitch whine or buzz heard from under the vehicle, dated 2-21-19.