I experienced wild stims quite often some times ago. There was quite rich "repertoire" of autostimulating behaviors. There was agitation wave. It was "pleasant", somewhat "euphoric". There can be walking without purpose, walking in the circle, running, jumping, various hand movements (such as clapping, flapping), screaming, saying words (sometimes repetitive, these words were rather often "funny"), laughing... It looks as I was "really" autistic. It looks "clearly" autistic. It could be really loud
And it was irritating to my grandmother and sister. It probably could be heard outside my house. My sister could tell me to leave the house because stimming was annoying. She is significantly younger than me, has about 13 years old, I am adult.
There can be many pleasant fantasies during stimming. Movements can be done "unconsciously", "involuntarily". It appears to be caused due to some sort of overwhelming, such as by anxiety (I named "agitation waves" as "euphoria" (or "something" similar) earlier, I was somewhat "surprised" that it may be caused by anxiety), obsessions, excitation (I had some agitation after reading that Lionel Messi might had been diagnosed with Asperger's syndrome, which was "beautiful" to my mentality).