If it really is coming from the speaker, then it's probably just interference. The speaker wires might just be too close to the monitor.
B+ is the main DC power supply voltage in a monitor. Input voltage gets rectified and filtered, then regulated to a stable, clean voltage. Usually something like 120v or so, but each monitor is different.
The flyback core is the ferrous core that's in the center of the flyback transformer. It's supposed to be secure, but sometimes in older monitors they start to get loose. They don't usually buzz thoug - it's usually a real high pitch whine.
But, if you are getting a buzz in the speaker, then it's not the monitor - it's probably just something interfereing with the speaker wires. Check to make sure they're not drooping down into the monitor or something, and that the grounds are secure.
-Ian