Without knowing what kind or how old he is, my first thoughts are it could be a cough or maybe a breeding noise? Has anything else changed with him or his environment? Is it all the time or just once or twice you heard it?
He has a coral adjacent to a very large fenced in backyard that I let him roam in. Late in the afternoon he stares in the large fenced in backyard and does that sound. When I put him in the large backyard he stops and begins his constent walking. Nelson is around six years old. I've been sooooo careful about keeping him warm etc.