Avada Kedavra and the Swishing Sound

[cindysphynx]

No: HPFGUIDX 30993

In GoF, we see Avada Kedavra used four times.  

1.  When Frank Bryce is killed, "there was a flash of green light, a 
rushing sound, and Frank Bryce crumpled."

2.  When Moody kills the spider, "there was a flash of blinding green 
light and a rushing sound, as though a vast, invisible something was 
soaring through the air . . . "

3.  When Cedric is killed, it goes like this:

"From far away, above his head, he heard a high, cold voice 
say, "Kill the spare."
A swishing noise and a second voice, which screeched the words to the 
night:  "Avada Kedavra!"
A blast of green light blazed through Harry's eyelids . . ."

4.  When Voldemort tries to kill Harry, "a jet of green light issued 
from Voldemort's wand just as a jet of red light blasted from 
Harry's."  There's no mention of a swishing sound.

So what is this swishing sound?  With Bryce and the spider, it 
appears that the swishing sound is the sound of the spell being 
cast.  But with Cedric, the swish happens before the spell is even 
cast.  And when Voldemort casts the spell, there is no swishing noise 
reported at all (although this might be because Harry is busy 
shouting Expelliarmus).

If the swishing isn't coming from the Avada Kedavra spell itself when 
Cedric is killed, what is going on?  Is this just another glitch in 
the graveyard scene?

Cindy