If the only time the buzzing happens is when the charger is connected, then it sounds like the charger may be causing the buzzing. If the charger was not causing the buzzing last week, maybe the charger is going bad? Or maybe the charger is closer that it was before and is causing interference. Another possibility is a motor of some kind running on the same circuit as your computer. For example, a fan, or refrigerator, etc. or even a florescent light can cause electrical interference.