Here are the links to threads that address this issue: 1, 2, 3, 4 (start reading after "double a" posts)
For those of you who own either XPS 12/13 - plug in your earbuds (preferably good ones) - try to volume up/volume down - after this activity your audio channel for headphones will be activated for a moment - and you'll be able to hear static/white noise in all its glory.
You can also hear it permanently if you delete your audio drivers. At first I thought it has to do with poor audio cable assembly inside the ultrabooks - it seems I've been wrong, every model has this. So this assumption is off the table.
Then what could be the possible reason? Idk, but this audio defect persists on every xps 12 model. And the sooner Dell fixes it - the better.
p.s. static noise is kinda like that