I haven't noticed this happening before... Today when I plugged into my Marshall 4x12 guitar cab is made a horrible buzzing mweeerrrrp sound when I played. It sounded like the entire cab was vibrating. Very bad. It only happened when I played with a bassier distortion and was much worse on certain notes / chords, notably A#. I wasn't pushing the amp too hard (reasonable volume) and was running the correct ohms from amp to cab. A friend said that the cab might be 'tuned' to a certain frequency and that the cab was sympathetically vibrating when I played near or at that frequency, but I don't remember that happening before plus that seems a bit crazy to me. Any ideas? Anyone know where else on the net to ask? Thanks in advance!