Perhaps someone who attends a lot of AB shows can answer the question.
At the end of the Bristol gig, when Mark put his guitar on the ground, there was a continuous noise (probably from the guitar?!?) - I have to say it was slightly annoying. Then, as he was leaving the stage, gave the guitar a huge slap/strum, then the noise stopped.
He strolled off with a huge grin on his face as if to say "That gets 'em everytime".