The defintion of the problem is called a ground loop. It is caused by the difference in potential of the two systems (phantom and gopro) as they each have a power source. The two are operating at different voltages, however the way the phantom is regulating the voltage to the gopro causes current to be pulled from one device to the other to effectively equalize the voltage between the two. When this condition occurs the interfrence is injected into the audio recording of the gopro.
Two ways to solve the issue, bring both power sources to the same exact potential or seperate the two so they dont effect each other. In this case the best solution is to isolate the gopro from the phantom power which it seams someof you have been successful at.
While i understand why DJI used the approach to supply power to the gopro it really is not nessasary and causes more problems than it solves. I make sure all my flight batteries and gopro batteries are fully charged so the likelyhood of the gopro running out of power is slim. Trying to supply power from the phantom for the gopro and resolving the issue would take a better design of regulation and a simple filter circuit added to the phantom power source for the camera.
Two ways to solve the issue, bring both power sources to the same exact potential or seperate the two so they dont effect each other. In this case the best solution is to isolate the gopro from the phantom power which it seams someof you have been successful at.
While i understand why DJI used the approach to supply power to the gopro it really is not nessasary and causes more problems than it solves. I make sure all my flight batteries and gopro batteries are fully charged so the likelyhood of the gopro running out of power is slim. Trying to supply power from the phantom for the gopro and resolving the issue would take a better design of regulation and a simple filter circuit added to the phantom power source for the camera.