Help with knocking sound

My truck has been sitting for for over a year.
I started building a new garage a while ago so the truck got ignored, but at least it got heated indoor storage.

This spring, I started back on the truck.... it needed a radiator, so while apart, I put in a new water pump, all belts hoses etc. Also put in fresh oil & filter.

Fast forward to today when got everything buttoned up & finally I started it (keep in mind I haven't heard it run for a year) & it has a loud knock.

The sound seems to come from the air compressor or that vicinity. I can feel it in the compressor & hear it through the radiator/dr side the loudest. I physically put my ear to the rocker covers & do not hear it.

It is impossible to isolate, as without it the injector pump does not turn so I can't run the engine.

Any ideas or possible issue with the accessory drive that could cause this?

Any advice is appreciated.

Pressurize the air system with shop air and see if the knock goes away or at least becomes not so loud...

I let it idle until it built full pressure. Sound did not change. You notice it less with increased RPM, but it sounds like a typical diesel knocking sound, except it is not on the passenger side.

I will pull it outside tomorrow to eliminate any strange building harmonics.

Oil pressure is good, no exhaust smoke or anything odd.

could be a injector.

I pulled it outside tonight & let it idle a while.
Although I still notice the sound, it is much quieter outside the garage.

The compressor still seems loud, but it got much quieter after the engine got warm.

I'm hoping that the new exhaust I just installed allowed me to concentrate on other noises that I never heard before.