I had something similar with my Rexton Cobalts (similar to Siemens Pure 700). I would describe it as persistent “S” and “CH” sounds. If the S or CH came at the beginning of a word then the SHHHH sound overwhelmed the rest of the word. If the S or CH sound was at the end of a word then there was a continuing SHHHH sound after the word. The word church sounded more like CHshhhhhCHshhhh.
I know this is probably not much help but the audiologist fixed it in about 2 minutes and it was not by lowering the gain. I stare at the treble, bass, and overall gain settings at every adjustment to make sure those do not get turned down in response to a request I might make. It might have been the compression she changed. It was something where there is a list of channels or bands (or something) with “settings” with numbers. If I remember correctly she changed the amount of some things on the list, not just clicking something on or off. Again, sorry, I know that’s not much help.