Jul 30, 2009 · I have a 2003 GMC Sierra 2500HD 4wd with the 8.1/Allison. Last weekend I took it camping and towed my pop up tent trailer. After a long and hot trip back home to Phoenix, where it was 110, when I pulled into the driveway I noticed a "hissing" sound coming from underneath the truck. He drives an '07 GMC Classic Duramax/Allison. He has in stalled the Banks 6 Gun w/PDA, 4" exhaust with banks muffler minus the cat, and banks cold air intake. He is experiencing a fairly loud whistling or blowing noise from idle up to around 30PSI boost. That is were it normally stops building power/boost and the noise stops.
Oct 02, 2019 · That is a good description of the noise and issue. My 2020 Denali 2500 6.6L makes the same noise. I assume other posts have a Duramax , so it’s not an engine issue. I found a technical bulletin regarding power steering noise on earlier models but it is related to extreme cold weather (-31 deg F), so maybe it is fluid related.