Not sure if I've even noticed that humming before! If I have, I probably thought it was part of the keyboard patch of something. Very cool.
I'm curious, the L2L section you posted reminded me. Are the "Woooooooa, ooooooh, hhooo" bits there at the start of Metropolis before the vocals start? (about 1:41) Like how they come through on the demo version, I swear I remember someone saying they're still there, just mixed really low. But I don't hear any vocals in Metropolis until "The smile of Dawn".