2003 GMC Sierra 1500 Pickup Owner's Manual - Page 419

6. Then replace the pressure cap. Be sure the pressure cap is hand-tight and fully seated. You may also hear this fan noise when you start the engine. It will go away as the fan clutch partially disengages. Power Steering Fluid 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 fan is spinning slower and the clutch is not fully engaged. This improves fuel economy and reduces fan noise. Under heavy vehicle loading, trailer towing and/or high outside temperatures, the fan speed increases as the clutch more fully 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. The fan will slow down when additional cooling is not required and the clutch disengages. 5-44 8100 V8 Engine