How to insulate DC from earth noise while keeping it referenced to it
All power supplies (AC mains to DC converters) generate noise at their output due to the fact that they use switching circuits inside to do the power conversion.

Some power supplies output less noise than others.

The large smoothing capacitor at the AC side of the power supply circuit does remove most of the noise of the AC mains but it will not and cannot remove all of it. Large capacitors always have inductive behavior above a certain frequency so filtering only happens up to a certain frequency.

Some power supplies have additional filtering at the AC side using capacitors and/or common mode chokes. This also helps to decrease noise being injected into the AC mains by the power supply!

The way how you measure can also have a significant influence on the amount of noise you measure. Is the power supply loaded or not? That might have an influence on how much noise the supply puts out.

How are you probing, via the ground clip on the scope probe? Note that the scope also has a mains earth connection which may form a loop and pickup noise.

Of course the design of the power supply matters, if it does not have a class Y capacitor in the circuit, the high frequency switching noise might "loop around" via the ground connection instead of taking the shortcut that the capacitor should have provided.