2004 Toyota Sienna white exhaust smoke

Just had head job, water pump, timing belt done on the engine. When it starts there a bellow of white smoke. Mechanic says the motor is good and thinks it is somewhere in the exhaust system. Catalytic converter, or clogged sytem some where.I read that the vacuum modulator might be a cause? Any ideas would be appreciated. Also, engine can idle all day and never over heats.