Minor picture interference query

[Spector]

I've got a composite modded Colecovision, and it's great. One I noticed though was that when there is no sound in a game, I can see faint vertical shadow lines moving in parallel across the screen. It's very obvious when playing a game like Video Hustler which has lots of gaps in the sound and a plain green playing area which shows the shadow lines moving quite obviously. In the majority of games it's no big deal, but I wonder what would cause that. When sound effects occur, the lines disappear, then come back when the sound stops. Again, I'm stressing it's faint, but my previous CV didn't have it. Anyone else recognise this effect?