I also have this issue with the SMART IX. I am very interested to know if you resolved this issue with firmware update or other configuration.
I also describe it as a hiss (shhhhh), that is present after the end of words, but more specifically anytime audio level input ends, in a low audio level listening environment. For example, if I snap my fingers, which is a short impulse input, I hear the noise following it, which is then gated/muted shortly after by HA algorithm. I’m concerned this is the noise floor of the device, and that I’m hearing the receiver output being enabled/disabled in response to the audio level at the mics. In a low level conversation, such as a quite meeting room, I hear the shhhh cut in and out as the audio level in the room is just loud enough to enable audio output from the HA. At least that’s what I perceive, with admitedly limited knowledge of how they work internally.