I had a similar problem with one of my home made power supplies.
The one shown in post #16 of this thread:
https://www.electro-tech-online.com...atile-bench-power-supply.127461/#post-1059392
The output was floating from earth and could cause problems in circuits under test, with a large AC voltage being developed between the 0v side of the supply and earth.
To fix the problem, I simply connected a capacitor and resistor in parallel, and connected them between the negative outlet terminal and the case of the PSU which is earthed.
The capacitor presents a low impedance to the stray AC, which is caused by the interwinding capacitance of the PSU.
The resistor prevents the build-up of charge on the capacitor, due to ??? who knows what effects in some future use.
The values of the resistor and capacitor?
Cannot remember, and it looks as though I did not go back and annotate the PSU schematic with the values either.
At a guess, the capacitor was a 1uf polyester type, and the resistor was 100k Ohm.
Have a bit of an experiment and see what works for you on your PSU.
JimB