Knocking sound due to rising oil temperature (3-3)
I believe the answer is:
rat-tat
'knocking sound' is the definition.
(I've seen this in another clue)
'rising oil temperature' is the wordplay.
'rising' is a reversal indicator (letters written upwards in a down clue).
'oil' becomes 'attar' (attar is a kind of oil).
'temperature' becomes 't' (abbreviation).
'attar' back-to-front is 'ratta'.
'ratta'+'t'='RAT-TAT'
'due to' acts as a link.
(Other definitions for rat-tat that I've seen before include "Sound of a knock on the door" , "Rapping sound of door knocker" , "Sharp sound of a door-knocker" , "Sharp noise of a knock on the door" , "Knocking sound" .)