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I tend to use humming or just “Eh,” when it comes to stuff like this most of the time. Occasionally I’ll us “Mm.” Or something similar. I really, really dislike it when people try to spell out those odd noises in books past that, as it’s just… funny. And weird. And it just kind of ruins the seriousness of a scene when your non-verbal character very seriously goes “Harrumph!”. Maybe if he’s an old, grumpy mentor type character it could work… but otherwise it throws off the tone for me.

In my opinion, keep it simple as possible. Saying “He made a noncommittal sound of agreement” is another way to not keep using “he grunted” over and over again as well. Snorting works well, but only in disdain or humor.