Do you have speakers near the TV? This is a common problem when unshielded (or poorly shielded) speakers are placed near a Tube TV. Many low-end Centre channels are often poorly shielded, often causing many problems.

If this is a new problem, you may be able to fix it simply by moving the speakers further away.

If not, it is relatively easy to fix, but you really should leave it to a professional, or you could end up moving the spot to the centre of the screen!!

Good Luck...

Derek

ps. Deal with this now...it will only get worse!! and possibly unrepairable.