I can't hear the noise myself but it kind of sounds like you either used to low a grade of gas or got some bad gas and that noise is pinging. Possible? Like I said I can't hear it for myself but the way it happens makes it a thought in my mind. Did you try draining the gas and putting fresh mid grade gas in?

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!