Engine sound possibly turned up on expensive car (4)
I believe the answer is:
purr
'engine sound' is the definition.
(I've seen this before)
'turned up on expensive car' is the wordplay.
'turned' is a reversal indicator.
'up on' becomes 'up' (similar in meaning).
'expensive car' becomes 'rr' (short for Rolls Royce).
'up' in reverse letter order is 'pu'.
'pu'+'rr'='PURR'
'possibly' acts as a link.
(Other definitions for purr that I've seen before include "Show pleasure" , "Low vibratory sound" , "Sound like happy cat" , "4's sound" , "Satisfaction from queen" .)