How long has the calf been doing this?
I have a calf who had a pretty bad bought with pneumonia for a bit. I almost lost him, but got him turned the corner. He still coughs some (though the cough is dry) and coughs during and after he sucks his bottle.
I have had bottle calves in the past that coughed after sucking their bottle too a couple of times.
If the calf is energetic about eating, clear eyes, and nose I would not lose any sleep over it. You can also monitor the temperature of your calf, since that is a pretty easy task in most bottle calves. I take temps in the mornings and evenings on calves I am suspicious are trying to come down with something. Anything over 103 I start to look at pretty close, though if they have been sitting in the sun all afternoon it is not uncommon to find 103.5 or so, which is why I take temps first thing in the morning or later in the evening. Certainly anything over 104 is probably going to get medicated, and the should be showing some symptoms at that time as well of respiratory or digestive upset and the overall dreariness that comes with being feverish.