Background:

No bees, but often some bumbling...what is a Buzz?  A rapid, really low frequency, relatively high (compared to music) amplitude sound that changes its frequency in a fairly random way. Whatever!  You know when you hear a "buzz" and it ain't musical!

Scenario :  A piece of the material in the ESL is loose a vibrating in response to a changing electrical field or mechanical disturbance in the speaker.  For example, you are playing some music at fairly high SPL and you hear a "buzz".  This can be caused by a panel vibrating against the frame in one corner.  The buzz may be a sliver of plastic tape or film that has been fixed in the speaker for years, that has now become a little loose as is whizzing back and forth in response to the vibration sin the dust covers.  Worst case, you have a small tear in a dust cover (or diaphragm) and the piece that is torn slightly is flapping in the breeze (so to speak).  Some buzzes are cured by taping and "padding" parts  of the frame with felt strips and the like.  Some require the dust covers to be replaced, or you can tape them over very temporarily with "sticky tape".  If it's a diaphragm fault, the panel has to be rebuilt, aaargh!