Air Pressure Noise???

By fflynn
July 8, 2004 in 1999-2013 Silverado 1500 & Sierra 1500

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fflynn

Posted July 8, 2004

Every now and again I hear a loud hissing noise, which sounds like pressure escaping from somewhere under the hood of my 99 Sierra Ext 4x4. It only happens when accelerating and mainly with the heater in the defrost position???

I brought it to the dealer... and as usual they were no help. I really have no idea what this is and it happens multiple times a day.

Anyone have any suggestions on what this might be.

Thanks

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