I dont know if anyone has said this yet, but one thing that I heard was that he only ever told one person, Robert Plant. And he has "since forgot." he said once in an interview another one of those amazing Zep mysteries

Im going to have to agree with Knebby, I just messed with it for a while, i definitely hear "_______ high hailla ball" (not quite sure what goes in the blank.) I think if the man asked Plant himself he is right, unless he is lying haha. im on your side Knebby