GMC Repair: 99 sierra hissing noise, gmc sierra, air intakes

Question
Hi Tim:

I have a 99 GMC Sierra with a 5.3 vortec. I get a really loud hissing noise every time I accelerate (usually about 2000 rpm). It seems the harder I am on the throttle the longer the noise sounds for. The strange thing is it stops after 45 minutes or so of driving. It also does it while parked and revving the engine. I tried to pin point it's location but it is SO loud that I really can't tell.

Thanks in advance,
Shane

Answer
do you have any air intakes or aftermarket parts on the vehicle, if so I would check those, if not I would check to make sure the tube coming off the throttle body is properly installed and clamped tight, also check to make sure the air box where the air filter is located is properly sealed shut