Engine Fan Noise - GMC Yukon Denalid Owner's Manual

4. With the coolant surge tank pressure cap off,
start the engine and let it run until you can
feel the upper radiator hose getting hot. Watch
out for the engine cooling fan.
By this time, the coolant level inside the
coolant surge tank may be lower. If the level is
lower, add more of the proper mixture to the
coolant surge tank until the level reaches
the FULL COLD mark.
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5. Then replace the pressure cap. Be sure the
pressure cap is hand-tight and fully seated.
6. Verify coolant level after engine is shut off
and the coolant is cold. If necessary, repeat
coolant fill procedure steps 1 through 6.

Engine Fan Noise

Your vehicle has electric cooling fans, you may
hear the fans spinning at low speed during
most everyday driving. The fans may turn off if no
cooling is required. Under heavy vehicle loading,
trailer towing, and/or high outside temperatures, or
if you are operating your air conditioning system,
the fans may change to high speed and you
may hear an increase in fan noise. This is normal
and indicates that the cooling system is
functioning properly. The fans will change to low
speed when additional cooling is no longer
required.