Noise or 'the Noise' is a disease which affects all human males and animals on New World. Words and pictures can all be seen and heard in the Noise. Settlers believed it was caused by the native humanoids, the Spackle. This lead to the slaughter of many of these creatures. However, the Noise germ was already in the atmosphere when the settler ships landed.
The men of New Elizabeth (now know as Prentisstown) started to become suspicious of the women of their town as they had no Noise. The women had realized by then that something horrible was going to happen and tried to escape through the swamp but many were caught by David Prentiss (later Mayor Prentiss) and were killed.
Later on (during the trilogy) Mayor Prentiss finds a way to utilise the Noise as an asset and use it as a weapon against people in many different ways, mostly doing so through mental exercises and repeating phrases such 'I AM THE CIRCLE AND THE CIRCLE IS ME' which can be heard from the Mayor's house at the start of 'The Knife of Never Letting go' and other various areas throughout the trilogy. In 'Monsters of Men' Todd Hewitt also utilises this power and uses it against the Mayor to free Haven.