In the case of my idler pulley, it was most every time I started up the car. and it would be a squealing noise that I could just hear over the engine (cold morning start up). but that would go away by the time I got to work.
I also replaced my power steering pump, because it was on its way out. It was making a whining noise, most notably in the morning. I had steering shudder, due to air in the lines. I had complete loss of PS at some points. Yea, it needed to go.