@smegel No problem I'm here to help not to fight

My guess is that the converter chips in the adapter are introducing noise into the USB power line and that the power filtering on the DAC side is insufficient to filter it out. The same problem is audible for me (to a lesser extent) with the HiBy FC6.
Just tested it again: ddHifi MFi06S is giving me a permanent mosquito sound on the left channel. While FiiO LT1 gives me a very quiet grill chirp on both channels.
Edit: the noise is permanent, even when no music is playing.
I'm currently using my Onix with the Cayin CS-L2C adapter without any noise problems on iPhone 14.