A loop is where a thread/rope/string, etc bends round so the string crosses over - like the handwritten letter 'e' - or almost crosses over or touches. A noose is much more specific - the string (or whatever) is in a loop, but tied, usually with a knot that can slide up and down the string, so if something is in the loop, and pulls, the loop gets smaller. This word is usually only used in the context of hanging a person (to kill them) - a 'hangman's noose'.
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