I would check the little bronze exhaust filters to see if clean, (remove for testing). Sounds to me like one is restricted and making the squeaky noise. Check them all too, for air to get in one side----air has to get out the other side.
The popping sound is the piston hitting the top of the release shaft because the air is not getting to the bottom side properly. During a tool change, air is send to the bottom of the piston first, then bled off as the air is sent to the top of the piston to release the tool. That way the piston does not slam down on the release bar, but goes down in a controlled (read quiet) manner. Yours is not bad at all yet. Can end up sounding like a hammer!
Trace the lines to the solenoid, which I think is on the top of the back. Clean or replace the exhaust filters there too. Hopefully the solenoids are okay.
Mike
Two Haas VF-2's, Haas HA5C, Haas HRT-9, Hardinge CHNC 1, Bother HS-300 Wire EDM, BobCAD V23, BobCAD V28