could be sign of a near failing head gasket, for now drive it like it is and observe coolant level and whether oil in reservoir bottle, and also observe engine oil level and whether if it turns milky, if either of these is present then its head gasket, if coolant is not reducing then a system pressure check is better to see where if any leaks are located. there is no other cause of white smoke apart from steam condensation on cold start, do you have thermostat installed? fan is direct or automatic?