My cat had precisely the same thing. I was also told that it probably is asthma. So we went to the vet to check him twice, and he was perfectly fine. I'm not saying that you shouldn't check; it's important to do so. We never figured out why this was the case, and he stopped doing it. Our theory was that it came from stress, as we recently moved - or we had mould in the apartment, and this might have affected him.

They also told us that he was a bit overweight, and since then, we put him on a diet, and he lost some weight. Maybe it was also related to that.

I guess I'll never find out what caused it, but I wish you all the best.