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:
The term is derived from the Greek words ὄνομα (onoma, name) and ποιεῖν (poiein, to make, to create). The same root as poet.