I'm so sorry to hear Coco is having medical problems. The first thing that came to mind when reading your post was tonsillitis. Maddie had that a few weeks ago and did the honking noises and also had white foam in her mouth and sometimes it would be white foam around her mouth. Did your vet look in her throat? If not, I'd say it could be a possibility she has a sore throat. YorkieUSA thought Maddie's symptoms sounded like tonsillitis and she gave me this link.
http://www.sniksnak.com/doghealth/tonsillitis.html
I hope you get it figured out and Coco gets well soon...Maddie sends kisses!