DroneUp which is mentioned in the article looks like a traditional quadcopter which goes mweeeeerrruuuuu:
https://www.youtube.com/watch?v=bK6bmQ6gX98

But the other mentioned Zipline is like a plane and more silent than street noises:
https://www.youtube.com/watch?v=__lhaW_yizk

If it flys higher, maybe it could be totally silent.