When you describe the ARB solenoid, I assume that you're talking about the electrically actuated solenoid that opens the line from the air resevoir to the differential.

Actually, it is rather rare for that component to fail. The "usual" ARB problems involve air lines being hit or snagged by rocks, tree roots, etc, and being ripped out. Also, sometimes the fitting where the line goes into the differential housing will leak, and sometimes the seal in the differential will leak. But, frankly, I've never seen an incident where the solenoid failed.

Rather than depend on your ear to detect where the leak is, I'd recommend getting a small squirt bottle (drug stores sell them for the ladies to put stuff in) and fill it with a capful of liquid dish soap and fill with water. Then squirt the soapy liquid on places where you suspect a leak. The ddramatic increase in observed bubbles will pinpoint your leak in no time.

If you're lucky, it will just be a line end or a fitting going into the solenoid that is the problem. If the "case" of the solenoid itself is leaking, then I'd call ARB and see what it takes to fix it. They may even replace the part, as long as it hasn't been hit.