When faced with a similar problem where I just could not find the offender, I had a tech equipped with a CB drive the node transmitting continuously while I monitored the headend. Found it, trightened the splice and it was gone. Interesting thing was that the cable swept fine, my best guess was the antenna of the CBer must have had a real good shot thru the back end of a splice that was only a little loose.
This is CABL.com posting #192888. Tiny Link: cabl.co/mYlg