You know, that happened to me, too. See, once, I decided to take my GameCube to my friend's house, because he REALLY wanted to play SSBM. Well, it turns out that his bedroom TV (the only set we were allowed to play on) had no A/V inputs. Luckily, I had brought along an R/F switch for that very reason. When we powered up the GCN, I noticed a sort of "wavy" noise pattern that should not have been there.
Maybe it's because the R/F modulator produced static during the RF/RCA conversion. Of course, it could also be an aftereffect of poor craftsmanship. I just don't know!