Engine Fan Noise - GMC 2007 Envoy Denali Owner's Manual

6. Start the engine and let it run until you can
feel the upper radiator hose getting hot.
Watch out for the engine cooling fan.
7. By this time, the coolant level inside the
radiator filler neck may be lower. If the level is
lower, add more of the proper DEX-COOL
coolant mixture through the filler neck until the
level reaches the base of the filler neck.
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8. Then replace the pressure cap. At any time
during this procedure if coolant begins to flow
out of the filler neck, reinstall the pressure
cap. Be sure the pressure cap is hand-tight
and fully seated.

Engine Fan Noise

Your vehicle has a clutched engine cooling fan.
When the clutch is engaged, the fan spins faster to
provide more air to cool the engine. In most
everyday driving conditions the clutch is not
engaged. This improves fuel economy and reduces
fan noise. Under heavy vehicle loading, trailer
towing, and/or high outside temperatures, the fan
speed increases when the clutch engages so
you may hear an increase in fan noise. This is
normal and should not be mistaken as the
transmission slipping or making extra shifts. It is
merely the cooling system functioning properly.
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The fan will slow down when additional cooling is
not required and the clutch disengages.
You may also hear this fan noise when you start
the engine. It will go away as the fan clutch
disengages.