First see where it's coming from, could be from the power supply, maybe a pop from the speaker, maybe the hard-drive spinning up or parking heads. Did you actually listen and narrow it down to the motherboard itself? If it's from there, contact ASUS support and see what they say.
It's definitely coming from the motherboard area since i took the PSU out of the case and the noise kept coming from inside, also i have no speakers, nor hard-drives.
Since there are no moving parts on the CPU and the tower cooler at this point it could either come from the Silentwings 2 Fan attached to the cooler or the motherboard itself