Gm has had an issue with the squeak noise coming from the steering racks. The noise travels up the column and almost sounds like its coming from higher up than the rack. The seal they use to seal the rack input shaft gets some deposits in it and hardens a little bit. On some of the vehicles the problem is so bad that the not offer just the seal housing instead of the entire steering rack and tell you to flush out all the P/S fluid in favor od Dex6 trans fluid (trans fluid is hydraulic fluid too....but with a **** ton more detergents to keep things clean).
Without looking at the vehicle, I cannot say for sure that is the issue. But from what you describe, that is where I would start my investigation.