As you're using glass CRT monitor as opposed to an LCD 'flat screen', it sounds like a magnetic field is nearby.

Loudspeakers could easily be causing the problem you describe. When CRT monitors were more common, computer loudspeakers had magnetic shielding to prevent such problmes, but LCD monitors don't need this shielding.

Some sort of external power supply (maybe the one belonging to your printer?) could also cause it.

You could try going into the monitor's menu and look for a de-gaussing option, which should correct any problems caused by such magnetic fields - but you must remove the offenders first!