I have had this problem since I bought the car in 2008. It is a 2006 ZX4. After reading this thread I remembered the noise when I bought the car. I told the dealership and they fixed it and told me it was the e-brake!!! I have replaced the shock and just a couple of days ago put a brand new swing arm (??) on at $145!!!!! and I just drove over the railroad tracks tonight and the sound is back!!! Sooooooooooooooooo pissed!
I am going to have my bf look at it and I am crossing my fingers because I simply cannot deal with the squeak anymore and it makes my car sound like a POS!!!