Starting to hear a low moan when turning wheels at stop

[Ghostrider]

The title kinda says it. Noticed a low moan while parked and turning the wheel left and right. I tried it in several parking lots and had it in all of them so it's not the tires and asphalt. It is very soft sound so there is no way to hear it while driving. Any thoughts on what to look at - power steering pump, belts etc..
I tried searching the forum and didn't find much

 

[JayRolla]

Low power steering fluid, air in the lines or power steering pump is starting to fail.

 

[BrandonF]

I've also been experiencing this noise for quite some time. My fluid level is fine, but I haven't tried flushing the system. Im assuming my PS pump is on it's way out. I've just been slightly lazy about it.