I was told its blowby of the o-rings in the pump,causing the impellers to cavitate making air bubbles

that causes it to moan as the air is compressed and does not have enuff pressure to work the load

so I was told by a hyd shop when I took mine apart and change the o-rings to stop it from leaking..which was were I dented the can and it would not seal

he then sold me a good pump and said dont take it apart

I get that too some times with my ?s