Not too long ago I asked the dr about my almost 15 month old doing a similar thing. He said its hard to tell whether its asthma or not unless you can actually listen while its happening and see if there is any wheezing. He also said to notice if it happens particularly when exerting himself, or if there is any other pattern. In the absence of any wheezing or anything, we concluded with my DS it was just a noise he likes to make. FWIW, he doesnt make it anymore, the phase seems to have passed!