Rental Income can be passive income as long as you don't have to work that much with the tenants and rental properties to earn the passive income.
To make passive income through rental properties you could hire a property manager to advertise the rental properties, process the rent payments and deal with upkeep of the rental properties as well as deal with the tenants.
Rental properties will always bring in money because everyone needs a place to live and not everyone can or wants to buy a home.