noise suppression methods for common mode 50Hz (power line) noise
Just as I suspected, you have a cheap 2-prong switch mode power supply.
To prevent electromagnetic interference (EMI), inside the power supply, there will most likely be a so called Y capacitor or EMI capacitor, between mains and secondary side.
This Y capacitor will make the output to not be completely isolated from mains, but to weakly follow the mains AC.
If you want less common mode noise, try another power supply, preferably one that has 3-prong mains input, so the output will have the capacitor to earth ground instead of mains voltage.