Coughing can mean a lot of things.

Sometimes kitties from the shelters get what is called 'kennel cough' (more common in dogs) and it is very hard to get rid of! There is usually a nasal discharge involved.

I'm glad the vet checked the heart because that offten leads to feline coughing. Did the vet check for Heartworm or just listen? Heartworms in cats offten leads to coughing and leathargy.

And it may just be hair balls. My cat makes horriable noises sometimes and then usualy leaves a mess somewhere. ICK! They make great tasting (to the cat) lubricants for hairballs.

Cheers,
T