Unlike the laptop itself, which makes various noises during operation, your AC adapter does its job in complete silence. Any noise emitted by your charger, whether it be a buzz, hum or clicking sound, indicates the charger has suffered some sort of damage and is malfunctioning. A faulty cord could cause damage to your laptop, so unplug a suspect cord from the machine immediately to prevent internal, and possibly costly, problems.
Cause
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Laptop adapters often suffer damage due to their typical placement and use. A cord stretched across a walkway is easily tripped over by people and pets alike, causing damage as it's yanked out of the laptop. Drops can damage the circuit boards located inside the brick, preventing proper conversion of the house current to the lower voltage necessary for the laptop. Power surges and liquid spills can cause short circuits inside the brick, producing smoke or strange noises when the cord is plugged in.
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Diagnosis
Unplug the buzzing cord and try a different outlet. If the buzzing persists, use a voltmeter to measure the output of the cord. Any reading that fluctuates more than a few volts from the advertised output on the brick's label indicates a faulty cord. While the cord is plugged in, feel the brick and determine if it feels hotter than normal. Check the cord itself for signs of burns, frayed wiring and severe kinks. Any of these symptoms indicate a malfunctioning adapter cord and requires replacement of the cord as soon as possible.
Replacement
Most laptop manufacturers offer a limited warranty on the battery and adapter cord included with your laptop, and if your machine is fairly new, you can contact the company for a possible replacement. For those with older laptops, new adapter cords are available at many online retailers or through the manufacturer's website. Generic, third-party adapters work just as well as the name brand versions and are often less expensive.
Required Wattage
On the underside of your laptop is a label with a listing of the machine's required volts and amps. Most adapter cords are advertised by watts, and to calculate the watts your laptop needs, multiply the volts and amps together. For example, a 19V, 3A laptop would require at least 57W to function. In terms of adapters, you can buy a cord with higher watts, but not lower. For the example, you could use a 60W, 75W or even 90W adapter for your laptop, but not a 45W one. Your laptop will only take as many watts as it needs, but it can't work — and may even suffer damage — with a cord that offers lower wattages.