What is the background sound during the outros on good kid m.A.A.d. city?

During the voicemails/talking parts on good kid m.A.A.d city, there's always a background sound. It sort of sounds like bicycle wheels. It also kind of sounds like old projectors, so maybe it's supposed to be imitating that since the album is somewhat like a movie? Or it could be on accident, however I feel like Kendrick wouldn't just keep a production error in unless it served some purpose. Sorry if this has been discussed already; I haven't been able to find anything about it.