power steering fluid

I hear a whirr sound when I am stil (not moving).i do not hear it when driving or when the car is moving and I turn...only when stopped. I thought I was just low on power steering fluid, but after topping off power steering fluid, I still have the sound. Is the only possibility a bad power...

Hey guys: I've had an intermittent screeching noise for the past few months and it took me a little while to narrow it down ... But I'm pretty sure it's the power steering pump (thanks to 232-K7 for his help). Process of elimination: determined it was an accessory by removing the serpentine...