Valid noise (5)
I believe the answer is:
sound
I believe this clue is a double definition.
'valid' is the first definition.
(similar in meaning)
'noise' is the second definition.
(I know that noise is a more specific form of the action sound)
(Other definitions for sound that I've seen before include "Speaking" , "note, for instance" , "Healthy; narrow channel" , "stretch of water" , "Fathom" .)