It's probably the air being pulled through the vent that runs vertically through the piston. Take a valve out and look at the bottom of it - see the small hole in the center? There's another in the upper side somewhere. and air goes through them on the valve downstroke and upstroke. Forcing some water through this passage may clean it of what's causing the chirp, but I've had better luck just reaming out the bottom hole a little larger.