My guess is that it is a laptop. Unplug the power going to the laptop and run it off battery, if this fixes your problem then get one of these.
http://www.amazon.com/Unknown-3-PRON...5590936&sr=8-1
You can find these at any store almost, Walmart, Home Depot, Lowes, Target, Radioshack, etc.
This will fix it if the noise is from a laptop plugged into the wall. My guess is this is it.
Bryan