I`ve finally found a solution to the howling noise my pc was making.
Audio demo: howling-noise (60dB gain from the original!)

Symptoms:

• Both soundcards on both speakersets had the same “noise” (meaning it couldn`t be the soundcard, cabling system or speakers!)
• Shutting down my sequencer stopped the noise
• The noise was periodic
• The soundcard`s controlpanel VUmeters lighten up so the problem was “in the box”.

The problem seemed to be in the box, next step was locating the problem.
It could be:

1. Routing scheme problem
2. Instrument
3. Effect

It seemed to be an instrument with effects on it. The effects on the channel seemed to cause the problem.