Dull impact sound (4)
I believe the answer is:
thud
'dull impact sound' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for thud that I've seen before include "Low dull sound as of blow" , "Dull bang" , "Dull, heavy sound" , "Dull heavy noise" , "Sound of heavy blow" .)