ok, I have a theory, the stereozoom in combination of compression and possibly sound recover is causing this problem. I believe compression or sound recover is adding a delay to the sounds from one or more of the 4 microphones prior to (or after?) it being combined together via stereozoom. the combination of the sounds maybe be causing certain sounds to be extended for a longer period of time.
to test this you got to be able to consistently reproduce this sound. I believe you just have to turn your head while hearing this hissing sound. The idea is that by turning your head you equalize the distant between the source with your aids. This will equalize or eliminate whatever delay there are, causing the hissing to disappear.
Of course, I could be completely missing the point here as I do not wear these aids.