I think I know the sound you are talking about. Its not a pop sound but a burble that keeps going if at a certain throttle setting. I had this sound on my Moki and it concerned me at first but it was determined to be ok. It wouldn't make the sound on the ground right? Your engine sounds fine to me. Just make sure you check the low idle "disk" every couple of flights and make sure it doesn't move on you. It sounds like you have a happy and strong Moki. Just make sure you are running a bit rich. When a Moki runs lean or hot it will die quickly, without warning.

hope this helps