Hi... here are some examples...
http://www.yorkietalk.com/forums/yor...d-trachea.html
Also, has his mouth and teeth been checked thoroughly? My Peek a Boo once had a hair or fiber that got snagged on on of his teeth, long enough that the rest of it went down his throat. He would cough and hack at the end as he tried to bring it up. I checked to see no hair was irritating his throat, but I checked again and found the string like thing stuck in his tooth, like dental floss.... when I pulled it out (looked like something pulled out of a drain...ewww!), there was bits of food and all kinds of stuff that had accumulated on it...