My dog goes nuts when I cough. Anyone know what might be a reason for this?

A great dog, a Labrador. Not phased by the usual noises animals don't like. He enjoys watching fireworks, and is indifferent to power tools being used near him, I recently rebuilt a chimney with him lying prone about three feet from the action, which involved a lump hammer and an angle grinder. He actually asks ( in his own way) to be hoovered when housework is being done, and also he is content for the petrol mower to p within inches of him, but cough and he runs for cover, demands to be let out regardless of the weather, is reluctant to come back in the house, and barks, which he hardly ever does normally. Help!