Why does my son have to constantly clear this throat when eating?

[Paul]

My 14yr old son has to constantly clear his throat when he is eating. He does not feel a need to do this at all when he isn't eating. I took him to our family doctor, and he gave him a quick look and said he probably has some mucus draining, but if that were the case, I would think this would make him do it all the time, not just when eating (it starts when he takes a bite, and continues until a minute or two after eating. Does anyone have an idea or understanding that might explain what is happening, or a suggestion about what I can do for him? Thanks.