Honda-SOHC

Hi, I own a CB550f 1977 and it has developed a ticking noise that increases with rpm. It sounds like it's coming from number 1 exhaust tappet. The strange thing is if I disconnect number 1 spark plug cap the ticking stops. Soon as I reconnect again it starts. I have installed new plugs and set gap correctly as well as the valves gap.
I have attached a link to a video so you can see/hear the problem. Any ideas or advice would be greatly appreciated.
https://youtu.be/HEqH2pH-4qU