Whistley, spitty sound

Sibilance?


It's not perfect, but it's all I can think of!!

Or spluttering?

Perhaps oristar, though the positioning of the mouth in your example would mean that the lips were pursed in the middle.

The references to "good side of his mouth" and "boyish and youthful" conjure up images in my mind of a kid with a wad of chewing gum tucked into one cheek and the noises I would expect to hear if he were trying to keep all the good, sugary saliva in his mouth, while talking at the same time.

Doh! Thanks JB, I misread the question....twice!