is this a HGJAD ? dont recognize HGFAD as a current ONAN model.
This is an EFI model, and fuel system works much like an automobile.
You have a high pressure pump, fuel pressure regulator, and return line to tank. Is "popping" coming from exhaust, intake, or ???.
would check rocker arms, valve guides, push rods. valve guides can come loose from heads, causing valves to not seat, and make a popping sound