I faced the same hiss when standing near a big running Mashall blutooth spk. It seems blutooth band interfere. Happen on the left.
Walk out some steps it disappears
Sorry, but no: if it where "Bluetooth interference", both buds would have it. Also, on this particular model, the left bud is not Bluetooth but NFMI.
I do believe the hiss is a result of something in the NFMI implementation, and made especially apparent by some quirk of the amplification setup used, but only Jabra can say for sure.
Also, having read complaints on other manufacturer's NFMI buds (MW07s, for example), I'm starting to wonder if this isn't common to all of them...