Noise is a loud or unpleasant sound. 
A noise is a sound that someone or something makes. 
Noise in an electric circuit is unwanted electrical signals. 
Noise refers to the unwanted parts of a situation, especially ones that make it more difficult for you to do or understand something. 
If someone makes noises of a particular kind about something, they say things that indicate their attitude to it in a rather indirect or vague way. 
Noise is an unexplained or unexpected information in a sample that is not useful and that can be ignored. 
Noise is an unnecessary information or activity.