My granddaughter has found that exact same noise (she is 10 months old). It's like they draw in air and make a growling type noise - - - well, not exactly growling, but it's a low toned "noise". She also went through a period of coughing, without a cold.
I think they like noises, especially when we react and laugh when they do it. But if you feel that he is not getting adequate air, please be sure to get him seen by a doctor.
my son started making gasping noises and coughing, a few days later he was wheezing and i took him to the doc. and he was diagnosed with asthma...... not trying to scare you or anything but just keep a close eye on him...it could be just a change in the environment!!!!!