I'm seems as if the weirdness evidenced when surface breathing new regs is more common than in the past. I have a SP G250HP and AL Cousteau. Both breath a little strange on the surface, but not when submerged where breathing "feel" becomes quite smooth. On the other hand, my old SP adjustable brass second which had always breathed smoothly up or down suddenly began to honk. There was also a slight freeflow. Turned out that the second stage spring was too weak.
Mulling it over, I'm beginning to think that the weak springs commonly used in balanced 2nd's have something to do with this strange surface performance. Possibly the resonance is damped when the diaphragm is reacting against water resistance.
Scubapro warns that their piston 1st stg may honk. They state that the piston stem O ring should be lubed and the HP seat replaced. However, I don't recall ever experiencing this particular problem.