A whistling noise can sometimes be heard when the Xstream first stage is pressurized or depressurized, and may be more noticeable on the surface or with lighter gases such as Trimix. This is due to a resonance frequency which takes place between the ball bearing and the actuating pin. This noise should not be detectable during low flow rates such as while breathing underwater. The noise has absolutely no effect on the regulator functionality. In most cases, the noise occurs on new or recently serviced units and will disappear with use.