Some manufacturers use insects (typically wasps or hornets) to test
their bulbs. Sometimes the insects are not pulled out of the bulb
before the bulb is sealed, and the end result is a buzzing bulb. You
can take the bulb back to the factory for a swap, or just live with it.
I recommend you just live with it - I took some back once and they
laughed at me, and then when I wouldn't leave and my facial tic started
acting up again, they called the cops on me.