Well if you got some really bad fuel, you could experience severe pinging. But your taking it easy on the gears. I could see it happen under more of a heavy load condition.
It is hard to describe sounds over the internet but I have been on bikes with severe pinging and it does sound like a "tinking" sound that kinda gets blown back but a gentle throttle usually will minimize it.
Before I went crazy trying to diagnose it, I would get a fresh tank of quality fuel and see if there is any change.