I think you’ve already go your answer here.
The difference between the duck head and the 3 prog cord is the grounding pin and its connection to a proper ground point.
I’ve seen similar issues with stereo speakers when the deck was not grounded you could hear the hum of the power supply (using a cheater plug).
While the new Power adapter may have additional shielding this shielding is useless until its tied to ground to dissipate the energy.
This gets into a floating [https://en.wikipedia.org/wiki/Faraday_cage|Faraday Cage] Vs grounded Faraday Cage.