Everyday talk is full of sounds that are 'in' speech but not 'of' it -- that is, sounds that we make which don't form words, but which contribute to participants' understanding of the interaction. Richard Ogden has been looking at how some of these sounds work in conversation, focusing especially on clicks (often transcribed as 'tut' or 'tsk') and more recently, swallows. Both of these often have visible as well as audible components. Their positioning in talk, which displays sensitivity to sequential, turn constructional and syntactic units, as well as elements of their phonetic and physiological design, provide participants with resources for both conveying something which is literally unspoken, and for making sense of what they see and hear.
Keevallik, L., & Ogden, R. (2020). Sounds on the margins of language, at the heart of interaction. Research on Language and Social Interaction, 53(1), 1–18. https://doi.org/10.1080/08351813.2020.1712961
Ogden, R. (2020). Audibly Not Saying Something with Clicks. Research on Language and Social Interaction, 53(1), 66–89. https://doi.org/10.1080/08351813.2011.619309
Ogden, R. (2021). Swallowing in Conversation. Frontiers in Communication, 6:657190. https://doi.org/10.3389/fcomm.2021.657190