We have a Carrier H/A heat pump unit. When the air is set for 78 and it reaches 78 and shuts off, there is a humming noise on the unit outside.

What is causing this noise? Is it okay? Is it normal?

The only way we can stop it is to turn off the system.

There is a reversing valve in a heat pump and it is possible that what you are hearing is the electric solenoid that holds the valve open