simple answer is that in less noisy periods, recording with auto will raise the base floor noise higher during those quiet times. have a manual/fixed level that's adjusted for your external mic will ensure you have a consistent level. Another main issue that people don't take into consideration is the mic distance to the source should be as close as possible and people expect mics to have some "reach". So when source distance changes (i.e. person speaking walks away from the camera/mic, the input level will decrease. In the case where there is constant variation of distance, auto will perform better but at the cost of more noise.
-jaewoo
Rebel XT, 7D, 5Dm3, 5DmIV (current), EOS R, EOS R5 (current)
