What Causes The Fluid In Mouth Of A Pneumonia Patient?

I m a nurse and have a patient that is having fluid coming from his mouth almost daily. He is not throwing it up and his lung sounds are clear but diminished. He has multiple health problems which do include COPD. He is on 4 L of oxygen continuously. This fluid is relatively new in onset and has started after a recent hospitalization for pneumonia. If not from his lungs, where is it coming from? YYYY@YYYY