Advice on Coleman Mach ac heat pump p

Just started camping season to find my roof unit doing a strange thing. Only way I can totally shut it down is by using the breaker. It will cool and heat but once it hits set temp the fan shuts down but a loud noise of something running in it continues for 20 minutes then shuts down. It does this in heat or cool mode. Never heard this before from this old unit. Anyone know why it is doing this? Thanks??