see: Echoing Black Holes, stating that the origin is the varying space distance between the Light horizon and the internal rim of the accretion disc around a black hole. also : https://www.sciencealert.com/eight-new-echoing-black-holes-have-been-found-in-the-milky-way
See below : the Penrose-Vuyk "Trapped Surface in Spacetime": or Repulsion Spacetime Horizon with variable distance between the accretion disc and the plasma producing trapped light horizon and electric charge differences between in- and outside .
Single Black Holes also seem to create a plasma cloud forming a plasma tail and accelerating the BH through the vacuum into a region away from other BHs like Ball Lightning seem to do. below.
