I don't know if others have had this issue, but on occasion the pressure relief valve on the engine driven compressor will leak. It seems to occur only when I bring my bus air pressure up via shop air instead of running the engine.

It makes a sound audible to my ears alone, but will stop after a while. None of my air systems (aux or the two braking systems) appear affected.

From trying to chase the source of that leak I know it sounded like it was coming from the radiator side of the engine, and it was only through the use of the stethoscope was I able to pin point it because apparently the sound echoes throughout the engine area.