Just as I was posting this, I realized that the only thing I hadn’t disconnected was my internet modem/router. It’s not near the amp, but it is in the same room. As soon as I switched it off the amp was dead silent. The click was gone. I’ve never had an issue with my old modem-router, but this one (a 2 in 1 modem-router) from my new internet provider is a whole different story. Thanks again everyone, that click was driving me mad.