Low continuous noise returning twice; very odd! (6)
I believe the answer is:
murmur
'low continuous noise' is the definition.
'murmur' can be an answer for 'noise' (I have seen 'Little noise' mean 'murmur' so perhaps 'noise' could also mean 'murmur'). I'm not certain of the 'low continuous' bit.
'returning twice very odd' is the wordplay.
'returning' is a reversal indicator.
'twice very' becomes 'rum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'odd' becomes 'rum' (rum can mean strange or dubious).
'rum'+'rum'='rumrum'
'rumrum' written backwards gives 'MURMUR'.
Can you help me to learn more?
(Other definitions for murmur that I've seen before include "Low indistinct sound" , "Abnormal sound made by heart" , "Indistinct mutter" , "Little noise" , "Quietly spoken utterance; heart defect" .)