i think it has something to do with the temperature, but i'm not entirely certain. i bought my car in may, and i NEVER heard a squeak until driving from vancouver (BC) to calgary in august.
i was doing an all-night white knuckler (heh) and about 4 or 5 in the morning the outside temp had dropped down to about 4 degrees celcius.
that's when i first heard the squeak.
then it went away again until this winter. i eagerly await the arrival of spring to see if the squeak goes away. it was 16 above here the other day, and i didn't hear it, but the temperature up here at this time of year is up and down SO much that it's hard for me to keep track of when it was cold or warm and if the squeak was there.
but when it's -30 it's guaranteed to squeak, but it seems the warmer it gets outside the less it squeaks.
i should say that mine squeaks only intermittantly, not all the time. i would say it's quiet about 75% of the time.