I have a 96 cummins n14 celect in a fld120. When ever the air compressor is engaged and pumping air my truck makes a ticking noise. As soon as the compressor kicks out the ticking stops. The noise does not sound like air ticking it sounds like a mechanical tick. It almost sounds like its coming from the air dryer on the passenger side not from the drivers side where the compressor is located. It could be coming from up front too. I replaced the dryer and it still does it. Its been doing this for months now its got me puzzled. Other than the ticking everything is working normal. Anyone have any ideas what causing this?

When the head gasket went bad and started to leak on the air compressor of my 02 Mack E7-427 it made exactly the sound you described. To find all I did was run the air pressure way down with it running, then with the compressor pumping I ran my hand all along the head of the compressor until I felt the pressure pulses.