How to eliminate noise from switched power supply
The core of an isolated switch mode power supply is still, though cleverly switched, a transformer.

Thus, your problem is the same as this circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

The whole point of an isolated supply is that there's not a single conductor from the primary to the secondary side.

Therefore, you can imagine the pair "OUTPUT+" / "OUTPUT GND" to have a defined voltage between them, but they float on  a "random" offset relative to the primary side's Ground. That offset doesn't actually contain much energy, and you can break it down very easily, for example by connecting "OUTPUT GND" to primary ground (thus eliminating the isolation!).