When I first heard Whole Lotta Love, on LP (or "vinyl" as they say nowadays for some reason), one bit that stood out was the way that Robert Plant's vocals were foreshadowed by a quiet, eerie repetition of the line that (unlike most repetitions) preceded the actual line, "Way down inside, woman, you need..."
You can hear it at the four minute mark here.
At first I thought that the line was sung so loud that I was hearing 'ghosting', something breaking through from the next groove, but I never heard a similar thing on "a vinyl". When I started reading books about Led Zeppelin, I got the impression that this was Jimmy's invention, and was called either backwards echo or pre-echo. They had statements like these:
...the track's famous "middle section." This was an abstract (but carefully rehearsed) gyre of sound - clamoring trains, women in orgasm, a napalm attack on the Mekong Delta, a steel mill just as the plant shut down. It had a strange, descending riff that Page sculpted with a metal slide treated with backwards echo.(Quite obviously no one else than Stephen Davis in Hammer of the Gods. A wonderful prose stylist even if the band didn't like his book!)
...the famous psychedelic middle-section of 'Whole Lotta Love', in which Plant's howling lust-maddened vocal improvs are mixed with an other-wordly cacophony of special effects, from the backwards echo of the slide guitar to the grinding sound of a steel mill, orgasming women, even a napalm-bomb explosion...(The much later and yet eerily similar phrasing of Mick Wall, in When Giants Walked The Earth - perhaps the first and the second passages together only count as one citation.)
Drenched in reverb as it was, "Whole Lotta Love" featured Page's innovation of backward echo, which he had tried as far back as the Yardbirds and on the earlier "You Shook Me" against Glyn John's reservations that "Jimmy, it can't be done."(George Case in the very readable and recommended Jimmy Page: Magus, Musician, Man.)
Pre-echo, baby. Run the tape backwards and print the echo to another track, then flip it and it comes first. Cool in both sound and concept.(geo, a person of the internet.)
It turns out, per the conversations in the WSJ that I, and many others, were reading something into it that wasn't there. The vocal is a print-through or bleed through from another track - but not on the record, on the master tape. It's not done deliberately by reversing the tape and rerecording it, it was just there already.
From the WSJ article:
Mr. Kramer: At the point where the song breaks and Robert slowly wails, "Way down inside…wo-man…you need…love," Jimmy and I heard this faint voice singing the lyric before Robert did on the master vocal track. Apparently Robert had done two different vocals, recording them on two different tracks. Even when I turned the volume down all the way on the track we didn't want, his powerful voice was bleeding through the console and onto the master.Some people today still think the faint voice was a pre-echo that we added on purpose for effect. It wasn't—it was an accident. Once Jimmy and I realized we had to live with it on the master, I looked at Jimmy, he looked at me and we both reached for the reverb knob at the same time and cracked up laughing. Our instincts were the same—to douse the faint, intruding voice in reverb so it sounded part of the master plan.
Mr. Page: I hadn't heard anything like that before and loved it. I was always looking for things like that when I recorded. That's the beauty of old recording equipment. Robert's faraway voice sounded otherworldly, like a spirit anticipating the vocal he was about to deliver.
Mr. Kramer: By adding reverb, we made his faint voice more dynamic, and it became part of rock history.
So you learn something every day!
The rest of the article is well worth a read too.