White noise is the fuzzy sound you hear when a radio is out of tune. Many LOs (and adults) find it helpful for sleep, it replaces the shush in shush/pat and drowns out all sudden or unusual noises around the house, outside traffic etc.
There are a range of different types of white noise, de-tuning a radio is one way, there are also machines you can buy, CDs you can buy, free down loads from the internet (to burn to disc and play on repeat). There are also things called pink noise, brown noise, lots of variety, things like water fall, hair drier, fan, trees blowing in the wind, sea crashing on the shore. All kind of droning waves of sound.
White noise (or any other colour) isn't considered a prop because once on it doesn't require the parent to keep returning to LO to resettle them.
Google it to hear some samples and get free downloads, also there are very cheap (like 50p) down loads on amazon and various other places, or you local music shop may have some CDs in stock.