A whistling noise can sometimes be heard when the Xstream first stage is pressurized or depressurized. This is due to a resonance frequency which takes place between the ball bearing and the actuating pin. This noise should however not be detectable during low flow rates such as while breathing.
The noise has absolutely no effect on the regulator functionality.
In most cases, the noise can be removed by turning the actuating pin a quarter turn. Contact your nearest Poseidon service center and they can help you with this.