Hi All - I'm new to this forum and was hoping someone out there might be able to help explain what's going on with my truck. I'm hearing a high pitched "squeal" when my truck is decelerating. My truck is a 2021 GMC Sierra 1500 w/ 3L Duramax i6 diesel engine and w/ automatic transmission (not sure of the transmission model). So far I've noticed the noise happens under the following conditions:

• after the engine has reached running temp
• only when the truck is breaking or "coasting" - by coasting I mean foot off gas but still in gear, not coasting in neutral
• only seems to happen (or at least only seems to be audible in the cab) between 1000 and 2000 RPM - it's loudest when it's closer to 2000 RPM and is no longer audible when it drops below 1000 RPM - doesn't seem to matter which gear I'm in
• the noise cuts out when I press on the accelerator - even if I just tap the "gas pedal" the noise will stop for a brief instant before it starts again
• it doesn't seem to happen when I'm using cruise control
• it only seems to happen when the truck is in gear - in other words it doesn't happen if I just rev the engine above 2000 RPM and let it come back down

I'm guessing it has something to do with my automatic transmission, maybe something with the clutch(es), but I was hoping to get a few more insights before I take it to the dealership.
Thanks in advance.
-Scott