Placing your hand within half an inch of your microphone. If the buzz becomes louder, the microphone is defective. You should replace your voice tube (CS70) or the headset (CS70N).
Plugging your base power supply directly into the wall if you currently have it plugged into a power strip.
Moving the base around on your desk. If the buzz changes as you move it around, it is possible that something is causing interference. Try to determine the source of interference. You may want to try turning off the electronics on your desk. If the buzz disappears when you turn off a device, try plugging that device or the headset power supply into a different outlet.