Buzz with all power off

Digital Cell phones (Blackberry in particular) can induce current on wires. Happens in conference calls all the time. Are you in an apartment next to a neighbor with a blackberry?

If within the time the buzz will increase, consider your speakers turned onto beehive bzzzzzzzzz LOL!

Do your high efficiency speaker have powered woofers?

Poltergeist. Seriously, I have noticed some high frequency noise from having radio devices (cell phone, modems, wi-fi) in proximity to speakers. Especially if the speaker wires are long. If the speaker wires are disconnected from the electronics and tracing it upstream and downstream may help.

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.

Check if there's wasp or bee or fly inside the speaker cabinette.