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Wouldn't have guessed exhaust leak either from the sound of it. Interesting. Glad you got it figured out.

Check all the hose clamps and connections between the turbo and the intercooler (the black pipe at the bottom of your picture). Also check for a crack or leak in the vacuum line on the wastegate, just left of the green outline. If you have some starting fluid.or propane you can carefully fog...

I don't have it setup on mine, I'm in hill country for sure and it's something I need to do as the brakes on the obs trucks suck big time. That's why god invented the manual transmission.

Ford uses an exhaust backpressure valve to warm the engine. It makes a similar hissing noise. It can be turned into an axhaust brake with a simple on off switch.

I'll second the turbo boot leaking. Is it low on power? Also does the isuzu have an automatic exhaust brake? I'm not familiar with isuzu's specifically but drive fusos for a few years. Another thing that might be worth doing is tilting the cab forward to pinpoint the noise or feel for air leaks.