Heard that the smell of vaginal discharge can change to a metallic sort of smell when a period is oncoming? is this true?

Uncertain: The scent of the vagina will change somewhat throughout the month and is influenced many there factors. I would not draw any conclusions regarding the menstrual cycle by a metallic smell. If you feel the odor is abnormal or unpleasant then I would schedule a visit for testing. Bacterial vaginosis is the most common cause of vaginal odor.