Airleak mystery

Id pay a shop to just use a sound detector on it if you absolutely cant find it. Its like $200 tops and saves several hours of going over it by hand. Most shops can find it in 15 mins or less if they are good and just charge you for that.

You can also buy them but....i think i paid like $2800 for mine. It has however paid for itself many times over.

 

"i head loud air leak near the drive axle"
If it is loud you should be able to pinpoint the leak.
"when I am not loaded....so I know it's air tight"
Possible that a line is getting kinked or pinched when loaded.
Should also check the trailer brake chamber pancakes as if one is leaking it can bleed back and leak out the dash valve.
See post #5 in this thread.
Air loss through dash valves when trailer is loaded | TruckersReport.com Trucking Forum | #1 CDL Truck Driver Message Board (thetruckersreport.com)

 

If your compressor can’t keep up it should be easy to hear when you shut truck off

 

You hear air flow through the dash valve because you have a leak in the air bag system. Find and repair the air leak near the the drive axles.