Discover the truth about low noise (6)
I believe the answer is:
rumble
I believe this clue is a double definition.
'discover the truth about' is the first definition.
(I've seen this before)
'low noise' is the second definition.
(I've seen this before)
(Other definitions for rumble that I've seen before include "Low, vibrating (traffic) noise" , "Thunder, perhaps" , "Discover the truth about" , "Find out the truth" , "Utter in deep voice, of have a slang fight - in the jungle?" .)