Hello,
I had the same problem and managed to fix it. Unfortunately most likely you will have a hard time fixing it because Apple 🍏 are *****.
Although the noise comes from the speakers, there is a 99% chance its caused by your display.
In my case I changed the top-case twice, didn't do anything and the third time I changed the display which solved the problem.
Your main issue now is that this problem cannot be detected by any hardware or software tests. and thats why the guys from the apple services wont accept the fact that your laptop has a problem. If its not detectable by hardware or software tests, this guys ( from the authorised apple services) wont get their money back from Apple for fixing your mbp ( if it still got coverage).
I had huge arguing situations with them and with apple care.
My solution was: 1. Call Apple Care and open a case on this problem.
2. Take a video of the popping sound and upload it on your case.
3. Force your senior advisor to send the video further to the apple technicians department
4. Wait for the technicians solution and than if they decide that your display needs to be changed, it will be covered by apple.
I bought my mbp 9 months ago. Repaired it 5 times.( different issues ). Now it got changed for a new one. ( 3 days ago ) because I had to repair it again. Out of 9 months my laptop was in service for 3 months and apple didn't give a ****. no compensation nothing.:) but that's another story
Still I managed to solve the popping problem on the old one by changing the display.
Hope for the best
Good luck