I’m just amazed at how clear the Led Zeppelin words are. But I can’t imagine what part played forward says six-six-six (I don’t hear anything repeated 3 times when listening to it forward).
The Beatles track sounds as though they purposely learned how to say that phase backwards. I once saw on an old episode of “That’s Incredible” in the 80’s where some guy could say almost any word backwards perfectly.
I heard a rumor that on Weird Al’s song “King of Suede” that he says “Satan eats cheese wiz” backwards in some part of it. I’ve never been able to confirm this though.