Was he/she in the water recently ? If so they might have aspirated and you would need to tip him over nose doen at a slight angle to let more water come out. If that wasn't the cause and the bubbles continue he might have an R.I.
Is he wheezing or breathing heavy ?
I do bathe her from time to time, so it is possible that she might have aspirated some water. I have been noticing these bubbles for some time along with some slight difficulty breathing occasionally but not all the time. I noticed also that her mouth is slightly open while she is sleeping. No wheezing, what do you think is happening?