Blood-streaked saliva once or twice a day, mainly on waking, with no pain and no other symptoms for a year usually comes from a small bleed in the nose or throat lining overnight, from dryness or mild irritation, or from the gums bleeding without you noticing while asleep.
Next Steps
See a dentist first to rule out the gums, then an ENT doctor if that's clear, since they can look directly at your nose and throat. Keeping the room humidified at night and avoiding nose picking or hard brushing can help meanwhile.
Health Tips
See a doctor sooner if the amount increases, you start coughing up blood, or you notice weight loss, night sweats or breathlessness.