I'm having the sound too, been there for past 2 yrs or so. Noticed that when I first start off the car, its fine, the sound usually starts after then engine has been running for a while. To mee this definitely feels like engine ping.
I think what bro Jazz said is correct, "cause by bad ratio mixture of fuel and air which indirectly incur bad combustion", feels like the car is running in a higher gear than it should for the current speed of the car, which is probably why it happens more at lower speeds.
So any other feedback on how to get rid of this sound.... thanks for all the inputs.