I have a 2004 Kia Rio and most of the time it works just fine, but every now and then the car won't start at all. I turn the key and all the electrical components come on just fine (lights, radio, etc) but its like the starter won't engage at all. There will be a click and hum for maybe 3-5 seconds but then it stops. Some times if I wait 10 minute ad try again it will work, but some times it will be 5-6 hours before I can get the car to start. I have had the starter replaced but the problem still happens, I am not sure where else to look at to fix this issue. Any help would be nice.

Does sound like the starter still. This is what I call a lottery start it is pot luck if the car starts each morning. The motor occasionally refuses to start to turn first time and has a defined rhythmic pulse when turning over - this is due to either burnt out armature coil or the contact face of the armature has become badly pitted or has worn off centre causing the brushes to lose contact.

? - replace brushes and ensure brush passages allow free movement

? - replace/ exchange the starter motor as armature contact area and windings are beyond repair.

If is just worn brushes another check is hit the outside of the starter motor whilst at the same time trying to start the car. If as you hit it the starter jumps into life and does its job then this is sign of worn brushes. There is no harm in taking the motor and starter solenoid out for a strip down and full inspection and possible clean up. To eliminate from the possibilities is a step forward.