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The air dryer is a Haldex Pureair. Parts are available at big truck stores, including a whole new unit. Erik's has parts for it:
Air Dryer Filter & Small Parts Kit For M35A3, Haldex "Pure Air Plus", DQ6026
My money is on the dryer also. In addition for trouble shooting purposes turn off the air assist steering valve on the firewall, just to make sure it is not the problem. The air assist vents into the air intake I think, and air escaping can do it quietly.