How do you spell it when make the sound for knowing answer?

The sound you're referring to is often represented by "uh-huh" or "mm-hmm".

These are considered affirmative responses that convey understanding or agreement.

Here are some other ways to spell it:

* Mmhm

* Uh-hum

* Mmm

* Yeah (informal)

* Yup (informal)

The best way to spell it depends on the context and your personal preference.