Sudden noise? Exactly (4)
I believe the answer is:
bang
This clue is a double definition.
'sudden noise?' is the first definition.
'exactly' is the second definition.
(thesaurus)
(Other definitions for bang that I've seen before include "Sudden impact" , "Strike violently" , "Explosive noise" , "Report - something the hairdresser did?" , "Sudden loud noise (often 'big')" .)