Whistling sound when turning

My father in-law has a 2003 (of mybe 04) Chevy pickup that makes a "whistling sound" when turning, it has done this for a while. Now my wife's '02 Suburban has starting doing he same thing. Any ideas what this might be?

Thanks

could be the power steering pump is low or going bad. i would check that and maybe drain and put fresh fluid it in.