I had the same problem with a wire, inside the speaker, vibrating in my left M60. Thanks to Joe's guidance, all I had to do was take the rear panel with the binding posts off, pull out the crossover, and try to gently move the wire a bit. I moved it enough were the vibration is only audible (barely) while playing at much higher levels than I usually do, and only when I put my ear directly behind the speaker.
If it's not a loose screw on the speaker, it's most likely a wire inside vibrating on the cabinet.
