Gulping noise when son swallows - is this normal

My son is 2.5 years old and has had asthma for 2 years. He has preventer inhaler twice a day and reliever when he needs it. He spends most of the colder months on antibiotics, and occasionally steroids and is on paediasure plus to boost his weight as he is underweight. For the past month he has started making load gulping noises when he swallows, as if he is swallowing a lot of phlegm. Sometimes he is sick and brings the phlegm up. Took him to doctor who gave him antibiotics which helped but now that they have finished, the gulping is back again (along with a nasty cough and a fever). I know if I take him back to doctors it'll be more antibiotics which I really don't want as they just don't seem to help anymore. Is this gulping normal and is there anything I can do about it. He's due to see paediatrician in January but I just wondered if anyone had any advice in the meantime. x