Kitty coughing, help

My cat has a cough, she has been to the vet, had antibiotics,steroids, X rays, stool tests, blood tests etc. the X rays were sent to a specialist radiographer and the answer was the same - I Don't Know what's wrong with her!! she is not a young lady but almost 17, she grooms well, eats fairly well, is reasonably active for her age, she has lost a little weight. she coughs when she purrs heavy, which she does every time I pet her, she doesn't bring anything up, has no runny eyes, drooling or anything else just this dry persistent racking cough......has anybody any ideas.
so far she has cost us quite a lot of money at the vets for nothing.
So now I am asking all you experts for your ideas and suggestions.