It sounds like you have a vacuum leak. The hum is probably the vacuum pump which is in the vicinity of the right rear door but under the seat, or it is a audible leak inside the door at the actuator. If the locking system is losing vacuum, the actuators may take forever to complete locking and unlocking cycles. I would start by checking the right rear actuator for leaks. Start with a visible inspection of the hose, fittings, etc. Then apply a known good vacuum source and observe for function. The pump is probably OK, but it's not able to compensate for the size of the leak.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.