Mixed equipment grounding
The DPU01M-5 is a 1 watt unregulated voltage output device so this means that if the load current taken by your circuit is influenced by the signal at the input (quite a normal thing to happen I might add) then you will see signal artefacts on the power rails.

If you read the data sheet for the DPU01M-5 it has a load regulation value of 10% and, the smallprint says this: -


  4.Load regulation is measured from 20% to 100% rated load.


So, if your circuit is drawing a DC current of (say) 60% of the rated current of the DPU01M-5 and there is an AC content to the supply current of +/-40%, then the output voltage will "waver" around by 10% and this is the likely cause of some of what you see.