There is likely some RF signal being picked up by the transformer in the amp and its charging a cap in the amp until it discharges. The RF can be something as simple as truckers passing by and keying there mic or it could be coming in through the mains. I've herd full conversations from ham operators come out of my speakers with the amp turned off. I've also had my amp physically rattle from some RF signal that would arrive at just about the same time every day several times a day. As far as I could figure it was likely some thing to do with the smart meters and the way they trigger them to send data or something. Usually 5 short bursts that lasted around 3 seconds and make tic tic tic tic sound.