Did what You sugested. Noise is still there, it starts when I hit play and stays there even when the music stops. The intense of this noise is the same irrespective of volume i set. I've tried all firmwares available from Fiio and tested on diffrent tracks. My CIEMs are Lime Ears LE3SW, but I can hear it also on my HD25-1, of course it's much quiter.
If you can hear it on the HD25-1 then I'll assume it is not normal. LE3 isn't that sensitive to be able to be affected by background hiss either, consider how good SNR is on the X5. Even my Shure SE530, which is rated to be more sensitive than your LE3 doesn't pick up any noise from X5. I think there might be something wrong with your X5 unit. You might want to consider an exchange with the seller.