That's low frequency rumble caused by wind. Caught at the right angle it doesn't need to be very strong. Even a light breeze can cause this. Low frequencies from try rumble or exaust from vehicles can produce similar effect as can hand holding.
Same issue for me, humming + regular static hissing when using external wired mic...really annoying. Even post edit did not get rid of it.
see here: https://www.youtube.com/watch?v=oxiYmAjfb2k