Its caused by a mechanical vibration.
First probe around the HV area to see if your noise
goes away. You can also make a stethoscope out of an
old paper towel tube or the like to pin point it.
If the flyback has nuts on it usually a very slight tightening
will fix it. Dont tighten much or you will crack the core.
Also check it mechanically such as screws holding it to
the PCB or frame. If its between the PCB & FBT wedge it.
If you can push & make it go away you can also wedge it often.
Same deal with yokes but its almost always the FBT doing it on
a B&W. Once in a while it will be a hefty coil in the yoke ckt
vibrating.
On a color set you can add the pin ckt & switching power supplys
to the list of suspects..........
In any case all sets do it some worse than others. I cant hear it
unless its really bad. Thats what happens when you put a box
of rounds through a .44 magnum without ear plugs
73 Zeno
