Check if there's wasp or bee or fly inside the speaker cabinette.
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Could be faulty dimmer switch or refrigerator or washing machine or A/C unit? You will need to unplug, (not just turn off) all of your gear to see if any of it is causing it. Then try unplugging other items around your house.
What's on the same circuit as your equipment? Flip off that breaker and see if it is still buzzing. Could even be an outside source...ham radio, cd radio, cell tower, etc.
Have fun troubleshooting.