@IslandKraut , when pressurizing from tank off, that disconcerting honk is completely normal for every Jetstream or Xstream. The valve insert (the blue umbrella) is completely depressurized.
When you open to tank pressure, air rushes out of the system at full volume until the servo tube pressurizes through the little filter at the top of the valve insert. Once the servo tube and valve insert are pressurized, the sides of the insert bulge out and seal the multiple holes in the valve. In the meantime, the sides of the valve insert are fluttering in the breeze, creating that resonant noise.
If you open the tank valve slowly, it's worse, as the restriction at the tank valve makes it take longer to reach IP in the servo. It's another reason why there's that second o-ring in the 3960 first stage - it's all an effort to pressurize the servo mechanism quickly. That's why you really need to "slam open" Xstream tanks. With the Duration, of course, and 100% O2 for accelerated deco, you shouldn't do that, and just need to accept the longer honk.
As for it happening with a purge, that must be a fairly vigorous purge! The Xstream second stage gas delivery is huge, and gas expansion at the surface is similarly large: 123 --> 14.7 psi is a more than 8:1 expansion. Because the valve is so capable, it allows downstream IP in the servo tube to fall off precipitously, making it much like tank opening. Even though the Xstream first is capable of large flow, all that expansion at the surface means a bigger drop in hose pressure downstream near the second stage.
All completely normal, albeit different than other designs. At depth, gas expansion is much less (e.g., at 99 feet it's 183 --> 59 psi, or only 3x), so the flow is relatively less abrupt, and since the first stage is able to maintain dynamic IP, downstream IP is maintained and the honking should have no equivalent vibration at depth.
I've noticed a little chatter with a lightly tuned XStream near the surface during heavy breathing, but it disappears below 20 feet.
A fault for one reg design might be a plus for another reg that's certified to 600ft Norsok standards. You can help minimize it for recreational diving by running your IP at the low end of limit (116-123 psi).
