Constant Audio Popping Without Framelimit @ Audio

This problem happens with both HLE and LLE. Sometimes right away, or sometimes after a while, audio popping at a seemingly fixed interval sets in. These are just little pops which appear at different volumes and at different 'locations' in the audio channels, at maybe an interval of 3 seconds. SWetting framelimit to audio fixed it, mostly. I would however still like to know what causes this.

My specs:
Processor: i7 4770k @ 3,5-,9
GPU: MSI GTX 760 Twin Frozzr OC
6 GB RAM
Mobo: Asrock z87m Extreme 4
Externel sound card: Asus Xonar DX