Yeah grabbed some flac files and copied them over to both the internal storage and the card's storage and only had the hiss from the files on the card. I can hear it when at low volumes and when changing tracks.
Just here to say this issue is present in my unit too. There's no interference noise when listening from the internal storage, but the same can't be said when listening from the SD card. It's only noticeable when listening in levels 1-3 of volume, but as I'm using an IEM I can't crank up the sound much more than that without becoming deaf. The issue is firmware independent, tested with the colorful firmware it came with (the one from aliexpress), the 1.7 AGPTEK, and the 1.0 (last) RUIZU.
A perk I found: although the 1.0 (last) RUIZU firmware can't sort the files in the proper album order (it always sort in alphabetical order), if you acess them through the File icon and not the Music icon, the files will be displayed in the first to last creation order (this happens in one of my folders, but in an other one it doesn't, I don't know the reason).
Other notes: The X02 is not gapless, and the gap is worse when using the SD card.
If there are RUIZU devs out there, the suggestions I'd make for a new firmware would be 1. Fixing the sorting order for albums 2. Changing the main color from green to blue (blue is more popular) or something neutral like white (also, offering an option to change the main color in the settings). 3. Replacing the gradient background with a solid color, probably black, or a dark blue, it probably should be a neutral, unsaturated and dark color. (this one is kind of personal, so the best thing would be to leave an option to change the background. The optimal scenario would be: the firmware has a standard background, but if the system finds a file named background.jpg with the right resolution in the root directory, the firmware replaces the background). 4. The selection color in the main menu should be the same as the main color. In the current firmware, the selection color in the main menu is yellow, while the selection color in folders and files is green. 5. Adding a brightness adjustment setting, as it has been added in the AGPTEK version of this player. 6. Holding the M button while in the "Now Playing" section could turn off backlight (but maintain music playing). This would be useful for people who read lyrics, because it's annoying having to press any button every time the screen goes black, and selecting the "Always On" backlight timer option would waste too much battery. 7. When a song has a lyric, it's album art is not displayed. What is displayed before the art is the standard picture for audio files. An improvement would be to show the album art until lyrics begin.