Covering the ears -Some people with Autism have a hypersensitivity to sound
Hand flapping – Not sure why but a lot of people with Autism do it. I think it might be another sensory thing, or just a way of making sense of their world
Humming, noises, etc – Some kids don't have functional speech but still make noise. There's very rarely a child who is truly non-verbal – ie can't use their voices at all as it includes things like humming, crying, screaming, etc.
There are various reasons for that.
because its annoying to listen to sounds, they would rather be in a silent room than a rowdy cafeteria
They are very sensitive to loud noises and don't deal well with them, so they cover their ears.
It depends on the individual but I used to work with kids on the Assburger end of the spectrum and they had a tendency to be super sensitive to certain frequencies of sound.
They likened it to a sudden, angering confusion. I imagined it to be something like having a tooth drilled into unexpectedly or getting a titty twister / purple nurple.
The frequencies were random. It could be anything from the sound of chewing to sharp or loud noises.
If they have really bad processing skills it can also just be a simple way of temporarily cutting down on outside impetus.
People with autism often do not process sensory information normally. This commonly leads to sensory sensitivities, especially to noise and touch. Most autistic people with sound sensitivities dislike loud, high-pitched noises, and many sounds at once, but they can be hypersensitive to seemingly harmless sounds as well, like rustling paper. Another common sensory problem is poor filtering and organization. When neurotypicals are surrounded by lots of sensory stimuli, their brains filter out many irrelevant details. Their brains also organize sensory information and allow people to focus on what is the most relevant. For example, concentrating on the person you're listening to instead of the music playing in the background. The autistic brain does not filter and organize properly. Autistic people can find themselves assaulted by stimuli in high-stimulation places like grocery stores and malls. Because of these sensory issues, some autistic people cover their ears in order to block out an unpleasant sound, or too much noise. Autistic adults usually find more subtle ways of dealing with sensory issues, like wearing earplugs.