Air leak issue

Have you tried checking with truck off-key on around fan clutch connections? No air to it when key off only with key on after probably 5 minutes after key on. you've probably noticed your fan is engaged for around 5 minutes after startup then shuts off. At that point you would have air leak in that system if that 's the problem.

 

I had/have a bad fitting on the line that tee’s in from the transmission. But you can here it leak with key off. I wonder if it is just your suspension / air bags while driving functioning (levelling valve on and off) since you can’t hear anything with key off?

 

I had a same problem. Mine air fitting on the fan was leaking, replaced with new fitting and the problem went away.
Built up air pressure, make sure you climate controls are in off position and then turn the key on with truck not running. I was able to hear air leak from the fitting.

 

Driver's side of the engine a little ways behind the fuel filter is a module that has some electrical connectors and a 1/4 air supply line (yellow or purple color) that has been prone to leak.

There's also an air manifold mounted to the drivers side frame near the transmission with about 7 different sized air lines going to it that's notorious for leaking.

 

Bad levelling valve, air leak from airbag or connectors, many things it could be. Get a plastic spray bottle one third filled with soap and your butt under your tractor and start looking neighbour.