A shussssss sound

[JimT]

If your air intake system doesn't have any significant leaks then I would focus on the exhaust system. One place on the intake side to check is the little rubber/silicon elbow from the air intake to the air compressor. I had one once that was so damaged that it looked like someone scraped it with a cheese grater. Despite the looks and obvious leaking, it didn't significantly affect my boost pressures while driving. However if you pressurize the system to check for leaks it would be easy to find.

A common culprit is the exhaust bellows (the thing that looks like a flex pipe). They develop small cracks over time and will produce a sound like you describe. Sometimes, early on, it will come and go only when you hit bumps, but eventually it will become more consistent. If you haven't recently removed any other exhaust components then this could be your culprit.

If you recently demandated then it could also be one of the other joints leaking a little exhaust. Especially if you didn't put new gaskets on between the exhaust pipes. Being demandated this wouldn't be much of an issue, as long as you don't have a noticeable exhaust leak under the cab, which is a DOT violation.

Hope this helps point you find the problem.