If it is two lines, evenly dividing the screen it is normal. Those are wires inside the tube that do something to stabilize, or otherwise improve the picture. They will move with vibration, and also respond to the degaussing field that is applied when the monitor is turned on.
It is a little annoying, but when you know it is normal it does not bother you so much.
However, is it more than two lines?
The wires should stay put, except for when the monitor is bumped.