Usually it's an airlock in the cooling system. Might be caused by the thermostat sticking. Might simply be from using the heater after not using it all summer. But the first thing to do is, from stone cold, take the cap off the expansion tank, switch the heater to maximum heat, start the engine, and run it up to temperature (until the thermostat opens, the fans start and coolant starts to blow out of the tank) then replace the expansion tank pressure cap and, with a bit of luck, you'll have purged the airlock.