I often in children's literature come across the rrrrrrrrrrrrS when a plane take off and the bumpity-bump when someone falls, etc.
and I am wondering if these are called with a specific term? written sounds? transcribed sounds maybe?
Making a descriptive word out of the sound that something makes is called onomatopoeia:
Definition of ONOMATOPOEIA
1 : the naming of a thing or action by a vocal imitation of the sound associated with it (as buzz, hiss)
2 : the use of words whose sound suggests the sense
The term is derived from the Greek words ὄνομα (onoma, name) and ποιεῖν (poiein, to make, to create). The same root as poet.