It might be the waterpump and that might be leaking into the engine oil which isn't good at all. Or there may be an airlock in the system driving pressurised coolant out of the pressure cap on the expansion tank. If it is that, from completely cold, turn the car's heater on, take the pressure cap off the expansion, start the car and run it at idle until pressurised coolant starts spurting out of the expansion tank. Then, using gardening gloves, put the cap back on.