Cummins ISC low power problem

I'd bet a dollar to a doughnut that: If you open the engine compartment, run your hand around the 1" rubber hose on the passenger side of the engine from the bottom of the intake manifold to the air compressor, you will find a hold big enough to stick your finger in it. And It will probably be right next to where it connects to the air compressor.
I had the same symptoms, whistleing noise, lack of power and threw a lot of parts at it until someone on this forum gave me the above advice. $40 later and a new hose all is good.