Board Thread:False Info and Speculative Discussion/@comment-1962215-20140812025444/@comment-13965604-20140815035111

Interjections are words used to express strong feeling or sudden emotion. They are included in a sentence - usually at the start - to express a sentiment such as surprise, disgust, joy, excitement or enthusiasm

http://www.grammar-monster.com/lessons/interjections.htm

He chose to interject umm to convey confusion, and disgust.