Does noise help or hurt concentration?

For years, I've thought that low-level background noise can actually help concentration. Bart Kosko talks about this notion in his book Noise. There's also recent research suggesting that noise can improve cognitive performance in children with ADHD. However, physiologist Mark Andrews of the Lake Erie College of Osteopathic Medicine writes in the new issue of Scientific American Mind that noise can lead to stress which actually impairs learning and memory. Bottom line? When it comes to noise, YMMV. From SciAm:
Several studies have indicated that stress resulting from ongoing white noise can induce the release of cortisol, a hormone that helps to restore homeostasis in the body after a bad experience. Excess cortisol impairs function in the prefrontal cortex–an emotional learning center that helps to regulate “executive” functions such as planning, reasoning and impulse control. Some recent evidence indicates that the prefrontal cortex also stores short-term memories. Changes to this region, therefore, may disrupt a person’s capacity to think clearly and to retain information.
"How does background noise affect our concentration?" (Thanks, Marina Gorbis!)