Did you try plugging other unit, like a speaker system? If the hum still exist.
Try plugging your system to a different outlet location, could be your PSU or motherboard have grounding problem. Have your replace the cord? Move your audio cable away to power lines, or/and use something like the ferrite. http://www.radioshack.com/product/index.jsp?productId=2103222
These suggestions does not guaranty to fix the problem, at least it'll decrease the interference.