Are the PSU switchers?
No, simple linears.
Does your scope have a third-prong-ground pin on its Line cord that connects is cabinet to power-grid ground?
Yes, it has.
Is the scope plugged into the same power outlet as the PSUs?
Tried on the same and on a different one. Signal shown stays unchanged.
Do the PSUs have a ground pin on their line-cord?
No, they are rectifiers and filters I've put togteher.
Is the common terminal (negative?) on the PSUs connected to power grid ground?
Sorry if I understand you wrong, but both PSUs are dual so common en each one is 0V and are connected to each other and to the board. Not to the power grid ground. Should I?
I do not feel comfortable with mains as much I didn't feel well with my ex mother in law.
Methinks you have a ground loop.