Been having so many issues with hissing and static noise at almost all levels of volume on my Fiio K5 Pro, and from onboard audio. It’s persisted on all my iems, and is particularly bad on my FH3’s. My motherboard doesn’t have onboard Optical or Coax either. Im not sure if it will help but I just ordered a Douk USB to Coax/ Optical as well as one of those ifi Attenuator dongles. The hissing also changes volume when turning it up or down, if that helps diagnosing. Gonna update this and see if anything changes with the additions next week.
Could be just your IEMs being very sensitive (= efficient).
Do you have any other headphone to test with?