Buzz with all power off

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.

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

Do your high efficiency speaker have powered woofers?

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?