I explained this problem in a video so you can see with your own eyes the problem... touching the GND wire makes the problem go away, however touching the signal wire creates the problem. Even touching the signal wire from the plastic it still creates the problem.
I am using the circuit below (instead of 1k resistors I am using 10k resistors) to bring the speaker audio level to mic level. The speaker is from a digital piano where I play musics and I want to record them on my smartphone. This circuit was kindly provided by @JRE.
However today I discovered a weird behaviour with it: if I connect it to my smartphone (to record the sound on the mic line - TRRS) I hear a hum/hiss. However when I touch with my fingers the gnd or signal wire, it disappears. It took me hours to realize this was the problem. And it's 100% reproducible: if I touch any of the wires (signal or gnd) the noise goes away completely, if I remove my finger from any of the wires, the noise comes back.
I am using my smartphone on battery (not connected to any external power supply). I read on Google about ground issues, but how this simple circuit could cause it? And how can I fix it?
As requested on the comments: my digital piano is powered from the outlet (220V AC -> 12V DC -> input of piano). The smartphone is not connected to anything else (not usb, no charger... just its own battery). The only connection between the piano and the smartphone is the GND pin and MIC pin (of the TRRS connector inside the smartphone).