curious noise when turning on ignition.

[Max V]

My 1977 CB750A has an interesting condition and I was hoping someone might have an idea. When I turn on the ignition switch, occasionally I will her a loud pop-like a firecracker. The bike still runs, but what would cause this to happen and how can i stop it from happening? It does not happen every time, but often.

Thank you
Max

 

[dirtdigger]

Has to be an electrical arc