eeny, I'm not setting myself up as an expert here as I am not specifically child mental health qualified, but I am a social worker with experience of children.
I personally have never seen or known of a child of your ds' age with an actual 'mental illness' - obviously some will display extremely upsetting/odd behaviour if they have been seriously neglected or abused but clearly this is not relevant here!! What he is doing, I feel absolutely sure, is nothing to do with mental vulnerability.
As others have said, it is a totally normal thing to do - many, many toddlers do it. And I do agree too that his keeping on will have a bit to do with your attention to it.
Honestly, you can safely ignore this!
Just distract him if you can, and carry on ignoring if you can't.
What he is doing is partly experimenting, partly for your attention, and partly a way of a child without sophisticated communication skills expressing a feeling.
Really, don't worry. He sounds utterly normal and I do think you need to be aware not to transfer any understandable worries you may have re mental health issues. With a loving, caring, attentive mum like you to bring him up, and a stable home, I bet you that is more than enough to overcome any possible genetic propensity.