If I'm reading this page correctly
https://en.wikipedia.org/wiki/U-NII
the table "5 GHz (802.11a/h/j/n)" says frequencies in this range are approved for 5 GHz wifi (your application), which says to me that someone else may be using it for a purpose other than your cameras. What that would be is hard to guess, but a regular pulsing sounds like maybe some sort of wireless link keep-alive. I once had a video repeater that shared the band with my cordless phone (2 Ghz? 2.5 GHz? don't remember now), and the phone would produce a repeating pulse in the video, making the video useless.