97 expy eb 5.4 hissing sound

:banghead: i have a hissing sound on my expy it sound like its coming from the idle control valve but its not
i change the gasket and still the same problem
when i'm idleing i can hear the sound but if i press the gas it goes away but if u let go it comes back
and when i'm driving in the freeway no sound but if u let go of the gas it comes back
someone told me to disconect my IAC valve to see if it was the problem i did and the same hissing sound
any suggestions
[SIZE=4]a/c on or off no difference :shakehead i change my IAC Valve and same noise no change

right under the IAC valve, is a rubber boot that's part of the EGR system, these rot out and cause a vacuum leak.
did the CEL come on? if not, it should have, replace that boot and your troubles should go away
hth
Rick, Ore

right under the IAC valve, is a rubber boot that's part of the EGR system, these rot out and cause a vacuum leak.
did the CEL come on? if not, it should have, replace that boot and your troubles should go away
hth
Rick, Ore
THANKS I'LL CHECK IT TOMORROW
I JUST DROVE FROM LOS ANGELES TO SALEM, OREGON WITH THE HISSING SOUND NO PROBLEMS (1800 MILES)
NO CEL LIGHT ...JUST A LITTLE LACK OF POWER GOING UP HILL

i fixed the hissing sound
thanks bearcat329
it was the hose under the IAC that goes to the passanger side valve cover
it was a 5 minute job
thanks again

right under the IAC valve, is a rubber boot that's part of the EGR system, these rot out and cause a vacuum leak.
did the CEL come on? if not, it should have, replace that boot and your troubles should go away
hth
Rick, Ore
its not part of the EGR system its part of your Positive Crankcase Ventilation
(PCV)

DOH!!!! you're absolutly correct! my bad, got the 2 confused, but glad it fixed his prob.
thanks for the correction!
Rick, Ore