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