Strangest Phenomenon I've Seen All Year - What Is Going On Here?!?

Just a short video on one of the strangest things I've ever witnessed - a clothesline vibrating up & down while everything else is perfectly still & motionless. No wind whatsoever, no tree branches moving, no fences swaying, nothing but the clothesline vibrating, going up & down in a steady rhythm - so strange!

Early last week, 1st clear day after a large winter storm system moved through - maybe caused by an inversion? This was in Montana... If you have any ideas as to what caused this weirdness, I would love to hear your thoughts in the comments below.. What is going on here?!?