Sources of whisper, hushed, in Rolls Royce? Sound of engine (5)
I believe the answer is:
whirr
'sound of engine' is the definition.
'sources of whisper hushed in rolls royce?' is the wordplay.
'sources of' says to take the initial letters.
'rolls royce?' becomes 'rr'.
The initial letters of 'whisper hushed in' is 'whi'.
'whi'+'rr'='WHIRR'
(Other definitions for whirr that I've seen before include "Spin with hum" , "Sound of vibration" , "Turn" , "Sound of rapid spinning or vibrating" , "Sound of something rapidly rotating" .)