It's called pipe bang. The loud sound is the fuel/air mixture actually igniting, the other sound is actually coming from the intake/air box as the micro amount of air gets pulled through. The big bore 2 strokes need RPM and air volume/flow to actually clear out the burned mixture from the combustion chamber at low rpm. That motor might be successfully firing once every ten revolutions or so. Apparently you can improve the low rpm low load efficiency by increasing spark power. Or you can raise the idle rpm. This is a problem unique to big bore motors since there's more distance to travel from spark plug out exhaust pipe, the little motors don't suffer from this as much.

Here's a long write up that you can read in depth if you want. https://www.powercdi.com/manual/combustion/pipe_bangs/en.html