ddec 3-4 12.7 whistle while under load when building air.

I have a ddec 3-4 12.7. I’ve had a whistling noise for about two years that I’ve been trying to nail down. Yesterday I noticed that it will only do it when the compressor is building air. It will only happen under moderate to full power, not at idle. I have changed the hose from the intake to the compressor a few months ago with no change, still happens. Now I noticed it was only when building air. Any ideas? Is there a direct path for boost pressure through that intake hose out through the safety valve when the governor calls for air? That is the only thing I can think of.

 

I need to add one more thing that you should know.
The whistle noise is variable with your foot input. It’s not. Constant unless you’re hammer down.

 

Noise only occurs when compressor is building air. As soon as it purges the whistle stops.

 

that makes sense. It would have a direct path from the turbo to the air dryer when building air. Ill investigate. Thanks

 

I’m going to take a dive into it when I get home maybe it’ll put out enough air with the red knob pushed in at 1000-1200 rpm to find a leak with soapy water. I’ll give it a visual tomorrow also in the daylight.
Thanks again for the insight. I would have never even went in that direction.