I don't have any solution for your problem primarily because I'm experiencing the same issue. In my case it happens exactly between 2500-2800 no matter if accelerating or decelerating rpms.
It too sounds like very fast pinging, almost like valve pinging but slightly higher pitched and way too fast for valves.
It happens after the engine is warmed up and usually after highway driving and not it doesn't depend on my driving style.
My mods are simple (gutted uppipe, K&N drop in) and I only started noticing it after putting the K&N filter in.
What are your mods?