I feel and hear popping like noise

I have Hyundai ELANTRA (Y- 2012). I frequently feel and listen the popping like noise when the speed crosses 20KM/H and 40KM/H. It is more frequent when the speed is slowly slow down to these two speeds. However it doesn’t occur if speed is changed drastically in either speed up or slow down.

Is it something to do with transmission? I have already changed the spark plug but no improvement.

Any help would be much appreciated.

BR