Air release from compressor safety valve.

2016 Coronado glider, 12.7 Detroit series 60, bendix truflo 550 compressor.
This has been a persistent problem for a couple years now. Late last fall I decided to finally dig in to correct it. I was hearing an intermittent whistling noise but had no clue where it was coming from.
I thought it might be a boost leak so I cleaned boost hose connections and used new clamps And checked CAC for Leaks, No luck.
One day I realized driving across Texas that it only whistles under load, moderate boost, at just below 100psi on my air gauges(cut in pressure), and air pressure will not rise while whistling.
If you lift your foot and let the boost fall it will stop whistling and start building pressure. That definitely led me to the safety valve.
I stopped at freightliner and bought a new valve but still had the problem. I also used a solid plug temporarily In place of the valve to make sure it was the valve and it did not whistle.
Replaced governor. No luck.
Then replaced the discharge hose believing I’m dealing with a restriction. No luck.
Then onto the dryer. No luck.
The day after Christmas, I spun the rear main bearing(-24f) and have since replaced the engine with a complete reman which included a new/rebuilt compressor and governor so those are totally eliminated.

As of Today, that’s as far as I’ve gotten with it with the check valve being the next possible(maybe) restriction in line.
Is it possible for the check valve/s to be restricted? The old compressor with 833k on it did pass a little oil so could the check valve/s be coked up from oil?
Also there is plenty of air pressure coming from the tank line at the governor and I blew shop air through the line from the governor to dryer with no problem.
I’ve called a couple of shops and nobody has ever encountered anything like this.
Am I heading in the right direction as far as a restriction or not?
Operation of the truck isn’t a problem, I still have plenty of air pressure but I know the safety valve should not be leaking.

 

Gauges read normal. Cut- In just below 100psi cut-out right at 120psi.
Ad9 dryer. Simple old school.
Most shops I’ve been to don’t diagnose anything. They just spend your money. plus I could never replicate the problem. It’s totally random.

 

Can’t be a restriction between compressor and dryer. The discharge hose is new along with the dryer.

 

I am referring to the safety valve on the compressor head.