Whistling sound when turning

My wife's '02 Suburban makes a "whistling sound" when turning. My father in-law has a 2004 Chevy pickup and it has done this for a while, but my wife's vehicle just starting doing it. Any ideas what it might be?

Thanks

4X4?

There is nothing in the steering mechanism (that I know of) that would cause a wistle sound. Is it maybe a squeek like a break squeek?

Does it happen when turning both ways? Low speeds only?

The only thing I can think of is when you turn, pressure shifts to one side and causes the caliper to press down and squeek. This can be adjusted.

try to have someone outside and duplicate the noise and try to narrow down where its coming from.

If it happens when youre standing still and turn the wheel. Then its easy to locate. Could be Power steering pump. Check oil for power steering and if serviced recently make sure its bled properly.