Worrisome engine sound (6)
I believe the answer is:
rattle
'worrisome engine sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rattle that I've seen before include "Child's noisy toy" , "Make someone nervous or anxious" , "Child's toy; unsettle" , "Fluster" , "Baby's toy - conductor" .)