I am by no means an expert, but at one point Brian May had a fuzz box installed on his original Red Special design. Eventually he fell out of favor with it and placed black tape over the red button to blend in with the pickguard. I assume he has since had it removed. However, he stopped using the feature very early in his career and I doubt this is the sound to which you are referring. It probably has something to do with a specific effect pedal or amps he used. He is partial to VOX amps, vintage, and has a smaller guitar amp he uses that was contructed by Queen's bassist, John Deacon...affectionately dubbed the 'Deakie' amp.