Hi, thanks for the reply. When I spoke to Marshall they weren't interested in dealing directly with me and said to send it back to the dealer and let them sort it. Which is fine, except that guitarguitar think it's normal. This is what they say: "Like i say when you engage a button like the deep button it will always add more noise as you are literally boosting that part of the signal. There is buzzing there but this is an electrical item with a speaker in it. 99% of amps have some level of buzz or hum to them." What gets me is that the buzzing appears even when there isn't a guitar plugged into the amp, i.e. there is no input signal!