Perhaps this ~280kHz "50% PWM" is emi coming from the LCD screen of the unit ! I not sure how the screen is actually painting the picture. It perhaps does look like the flat noise being offset on the screen, which then looks like PWM ?
I had probe set to AC 10x with the ground clip on ground lug at the 1kHz terminal.
Move probe closer to screen, amplitude (peak of "PWM") goes up, move away from screen and amplitude drops off fast.
I wonder how much noise these Rigol "packages" make?
But duly noted, I could not reproduce the noise I saw on the PSU DC when just having a floating probe attached. I was able to see something small at around 1.5MHz, but I might suspect that's just the noise from PSU coupling over to the floating probe ("antenna").