GMC 2003 Sierra - Manual (Page 419)

T>ven lepiacB the
pressure cap. Be sure
the pressure cap is
hand-tight and
Fully seated.
Engine Fan Noise
Your vehicle has a clutched engine celling fan. When
the clutch is engaged, iFie Fan spins iaslerto provide
more air lo coot the erìgine. In most everyday driving
condilionsH tFie fan is spinning slower and the clutch is
not FuNy engaged. This Improves luel economy and
reduces Fan noise. Under heavy vehicle loading, irailer
[owing and/or high outside temperatures, the fan
spaed increases as the dulch more fully engages. So
you may hear an increase In fan noise. This is
normat and should hol be mistaken as the transmission
slipping or makrng extra shifts. Et is merely ihe cooling
syslem functioning property. The fan will slow dowrt
when additionat cooling is not required and ihe clulch
disengages.
You may also hear this Ian noise when you siart the
engine. It will go away as Ihe fan clulch partiallv
disengages.
Power Steering Fluid
aioo ve Engine
5-44