Air hissing only under load accelerating seems air compressor and boost related

The air compressor gets filtered air from the intake charge air cooler system. The leak you hear when pulling hard is either the hose going to the air compressor or a leak in the charge air cooler system. Have the charge air cooler system checked by taking apart the system where the turbo supplies air to the intake air charger cooler and block up the system by the intake manifold. Pressurize the system by adding air into the manifold where the manifold pressure gauge, just a simple 3/8th fitting and an air chuck screws right into it. Plumb a gauge with the air chuck and pressurize the system and see how much air you loose in 15 seconds. you should be able to hear the leak st this time. 2 or 3 lbs air loss is normal, but more than that you have problems. The engine oem usually say 5 lbs loss in 15 seconds is normal, but it is so they can get out of warranty work.

 

Carefully inspect the air compressor intake hose for damage or loose connections. Also check all of the pipes from the turbo outlet to the engine intake. If all seems OK, then perform the pressure test outlined by mnmover above.

 

A spray bottle with soapy water will find your leak

 

I'd think maybe the pop off valve on compressor or purge valve on dryer.

You said it quits when the compressor engages and starts building air again, even if on the throttle hard?

 

Check the pop of valve located on top of the compressor, check the purge valve under the air dryer

 

What dryer do you have?

 

I think, could be completely wrong, the electric plug on the purge valve is for heat, to keep the valve from freezing in the winter.

I would just change the pop off valve and the purge valve.