No glitching or crackling in the sound at all and I don't think it's sound related. Tapping on the machine itself doesn't replicate the lines, either.
And after looking again, it appears that it does in fact do it whenever something is moved on the screen. If it sits still in Workbench or on a game menu screen with no movement, it doesn't flicker. If I remember correctly, the problem doesn't exist on the A500 when using its' composite port with the same TV and cable (the A500 is an NTSC model)
At the moment, I am going to assume that it's composite related until I get a Kipper2k RGB to S-Video adapter.
Addition - I just noticed that once Worms gets into the main menu, the power light is noticeably dimmer. I am not sure if that is normal or not. I'll have a video in a bit to show it.