its usually trapped gasses in the coolant.
first bleed the cooling system with the heater set to full hot to allow any air to escape, rad cap off and run engine till the thermostat fully opens and rad fan has turned on then off.
fit the rad cap and take a test drive.
if it still sounds like its bubbling after turning the engine off, then combustion gas is still getting into the system which is usually the start of a blowing head gasket.
when the gasket blows bad enough, it can force coolant out of the expansion tank. no coolant, no cooling.
could get garage to test for exhaust gasses in the coolant to verify the HG is leaking before considering repairs