Did you look at the z screw? Is it rotating when this happens?
I have 10 theories that might cause this. I'm not excited about any of them. Lets try to eliminate some things. For example the Z stepper might be too weak to hold the bed up. If so you should be able to see it rotate on every "pop".
Or the linear bearings might be defective. Or the vertical friction might be extreme and the Z axis might move in a mixture of small and large steps. Or there could be a cura bug that is moving the Z on purpose or many other things. Or the Z nut might be loose and rotating.
Try to get some video of the z axis to see if it is rotating. Try to get some video of one of those 2 vertical rods that shows the movement when the "pop" happens.
for the rotation of the z axis it might be better to video from below the bed but don't let your phone or hands get crushed!
Also it would be good to show the extruder motor in a video while the popping occurs. I suspect the popping is caused by 2 different things:
1) Extruder motor
2) Something to do with the Z axis
And the videos you posted only showed #1 possibly. #1 could possibly continue after printing is stopped but only 2 or 3 times.
I'm sure it comes from the z-axis. Sometimes when it pops louder, the buildplate goes down about 3 or 4 mm and the UM begins to extrude material "in the air". In the first video, at 0:43 I interrupt the print job and the pops remain.
I also have the underextrusion issue (I managed to isolate the problems) and it prevents me from printing at speeds faster than 50mm/s (even with .1mm layer, 220C and UM original filament, for example), with the tension on the filament set to the minimum. But, while I don't have time to solve, I can cope with this by printing slower. Probably it's something related to the spring tension inside the extruder drive.
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