The sound of 'pong'? (3)
I believe the answer is:
hum
I believe this clue is a double definition.
'the sound' is the first definition.
(hum is a kind of sound)
''pong'?' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for hum that I've seen before include "Low sound of bad smell?" , "Pong" , "Insect sound of doubt" , "Smell badly (slang)" , "Drone - stink" .)