Our dog is coughing and gagging and spitting up white foam. Nothing is in it, just plain white foam. We took him to the vet today (for a different reason) and the vet asked if he had been coughing because this happened before and we said No because he hadn't coughed in months but as soon as we left the vet's office our dog started the coughing in the car. The funny thing is that the only times this has happened is when we have left him with my sister to care for. She has a black lab. Not sure that would have anything to do with it but it's funny that it happens after he comes home from her house. Our vet did say that our dog has a heart murmur and on a scale of 1-6 he is a 3. Any ideas?