GMC 1999 Sierra 1500 Pickup Owner's Manual page 259

Engine Fan Noise
This 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 every day
driving conditions the clutch is not engaged. This improves fuel economy and
reduces fan noise. Under heavy vehicle loading, trailer towing andlor 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. 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.
If a Tire Goes Fiat
It's unusual for a tire to "blow out" while you're driving, especially if you
maintain your tires properly. If air goes out of a tire, it's much more likely to
leak out slowly. But if you should ever have a "blowout," here are a few tips
about what to expect and what to do:
If a front tire fails, the flat tire will create a drag that pulls the vehicle toward
that side. Take your foot off the accelerator pedal and grip the steering wheel
firmly. Steer to maintain lane position, then gently brake to a stop well out of
the traffic lane.
A rear blowout, particularly on a curve, acts much like a skid and may
require the same correction you'd use in a skid. In any rear blowout, remove
your foot from the accelerator pedal. Get the'vehicle under control by steering
the way you want the vehicle to go. It may be very bumpy and noisy, but
you can still steer. Gently brake to a stop, well off the road if possible.
If your tire goes flat, the next part shows how to use your jacking equipment
to change a flat tire safely.
Changing a Flat Tire
If a tire goes flat, avoid further tire and wheel damage by driving slowly to a
level place. Turn on your hazard warning flashers.
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