ft has it ... a peculiar "rrrrrrrr" sound. I think the engine in question is the CFM-56 family. I have heard it early on approach and it is not noticable (or as noticable) on takeoff. Never figured out how to listen for it in cruise from the outside.
Believe this is a common engine in A319, A320, A330s as well as latest mark 737s.
It sounds almost like a gear box or something or maybe one of the mechs has a way to fasten baseball cards to the nacelle so they bang on a fan when engine is in operation.