Kboring is right. It does it on both my Denali and H2 and has done it on both from the beginning. The fan clutch is somewhat variable. It also does it when you first crank up in the morning as some type of test.
The clutch mechanism is not mechanical in nature but is a magnetically reactive fluid. GM calls the clutch "Magnetorheological Fluid (MRF) Fan Clutch". The fluid in the clutch changes viscosity to turn the fan on and off.
When the engine is at temp the viscosity of the fluid is at max and the fan is fully engaged and moving alot of air, that's what you are hearing.
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