I've been having this issue for the past few days, got my earphones in September and did not notice anything like that before.
It's very likely a firmware issue related to Sony's noise cancelling algorithms, probably arisen after some recent update. After wearing my earphones for about an hour in a moderately quiet environment (or even a complete silence) they start making a rustling noise in the right ear. I've tried making artificial noises, touching the right mic or blowing into it, and this didn't help much, although I think I noticed some differences in the noice after those manipulations.
A workaround fix that I've found for this is turning off the earphones for a few minutes (less than a minute doesn't seem to help), which makes the noise go away for another hour or so. This is probably related to some internal spectral stats buffers accumulating parasitic signal and then being reset. Starting the noise cancelling optimizer doesn't seem to reset those stats however, which is weird.
Hope Sony will be able to fix this in upcoming firmware updates. However, it's important that the community support forwards these complaints to a relevant team so they're aware of the problem and are able to reproduce it.
