Addressing your post title,
http://books.google.com/patents?id=6...X-tnh-dQ&hl=en
shows a page that discusses Barkhausen oscillations in a tube that can generate an annoying sound in audio equipment and a vertical line of hash on the screen--equally annoying--in video equipment.
A trick that sometimes stopped the annoyance in a TV set was placing a "beam bender" magnet from a cast-off CRT onto the horizontal output tube, and moving it around in all directions until the line disappears.