Is there any significance to the cough at the beginning of Led
Zeppelin's "Whole Lotta Love"?

As a huge Zeppelin fan, I've yet to hear any significance to the
'cough' leading off the track. In fact, I did not notice it there
until I saw it printed in the complete guitar tabulation of the song!

 During the early days of recording Zep albums, studios were still a
bit rudimentary as the music was only pressed for vinyl albums, played
on machines making the transition from mono to stereo. In addition,
radio was still very much an AM band dominated industry, at that
point.

 Also, guitarist Jimmy Page produced and mixed all Zep albums (but
one), who was not the most sober of perfectionists. In fact, such
audio bits can be heard throughout 'Physical Graffiti'.

That's interesting, I always thought it was Robert Plant laughing at
the beginning of that track.

Another interesting fact is that I have a bootleg of a finished demo
of the song that the band put out before recording the definitive
version. That cut opens with what I can only describe as a 3-4 second
plinking circus sound, like the kind you hear when you turn the crank
on a "Jack in the box". So they definitly had the idea for some sort
of intro to the song.