My '04 started making the same noise at about 100k miles. Very intermittent at first, two years and 30k later it is starting to make the noise very frequently. There are lots of hills in Western PA and it seemed to first occur only starting up a steep grade, it has slowly worsened and in the last few weeks it doesn't take much of a grade to generate he noise. I have managed to keep the noise to a minimum by using my parking brake. When starting on a hill, I keep the release button held down while I pull up on the brake arm. Don't let the car drift back, then as I release the clutch, lower the brake. The whole goal is prevent the car from drifting back and putting additional pressure on the clutch - simulating a level road surface. Mine doesn't squeak when I do this, or I should say didn't, it's starting to now, which is why I am going to be done with it and have the clutch replaced.
I have not seen anyone report a known exact cause, although I have read that people said a new clutch and t/o bearing kit fixed it. Either way, there is no easy pill for this one. The only way to assess the issue is to pull the transmission. I believe the estimate to R&R the trans is 8-10 hours of labor. Every situation is a little different, but my odometer is 130k, so it would be a mistake to spend $750 to $1,000 and not replace the clutch for another $300. These cars are really reliable, so I am confident that my car could easily see 200k.
I will try to remember to update the board, but my guess is that the answer will be what no one wants to hear.