My understanding is that what we are doing by nebulizing is working moisture into our lungs to help loosen the mucus. I believe that what does come up as white foamy bubbly is the moisture we put into the lungs to loosen the mucus and hopefully also mucus with it. I maybe wrong but we are probably bringing up some mucus and moisture at the same time. An infectious disease doctor did say to me, when I showed him the urine specimen cup I carry around with me in case I have to clear my throat, the white, foamy, bubbly, "is moisture". So for me my conclusion is/was mucus and moisture that comes up is due to the nebulizing to bring up the sputum/mucus with our huff coughing to clear it out immediately after nebulizing and/or during the day. Yes, for me it is an almost all day thing of having to clear my throat of sputum/moisture/mucus.
Had you asked your pulmonologist what he/she thinks it is? Other than the Infectious Disease doctor no pulmonologist doctor indicated or informed me that it is "moisture" when I showed them the same thing, they had no comment. Go figure.
Barbara