Power Steering Noise after Car is warm

After my 2007 Acadia is driven a few miles, when I turn the steering wheel to get into a parking space, there is a noise which sounds like something is binding or scraping. I have taken this car to the dealer twice, and they claim they fixed it (by refilling power steering fluid to remove air in the line according to them), but the same problem returned. Any ideas on what is the cause and how to fix this? Is this problem a safety risk?

Try another mechanic.

Thanks - will consider that option.