It depends. It isn’t just kids with autism that do this either, so it isn’t an “autistic phenomenon”. If there are things that are scary or just too visually overwhelming, they will cover their eyes. If they feel embarrassed, angry, sad or shy, they may cover their faces. If there’s a noise that’s too loud or a noise they are unnaturally afraid of, e.g., a vacuum, they may cover their ears.
Because kids with autism are already hypersensitive to everything around them, they are trying to block out the offending sensory input. It bothers them, and everyone around them needs to be considerate of that. That doesn’t mean everyone has to tiptoe around them; it just means they can’t purposely annoy them with an annoying sensory stimulus.
Kids that appear to do this a lot, autism or no, there is a way to help them through it. They need to see the object that causes the noise. They need to touch it when it isn’t making noise and then, once they are used to that, you can turn it on. Children might freak out at this point, or might be more accepting of the object. Not moving the object and letting them touch the still item will help them become acquainted with it. The final step is to move the object, in this case, a vacuum. (This approach may not always work because you’re certainly not going to move a plane or helicopter.)
This gradual introduction to the sensory stimulus that scares them or makes them nervous will help them overcome it. In turn, it decreases the number of times children will cover their faces, eyes, and ears. On a psychological level, this is known as the process of desensitization. Just as people become desensitized to violence by watching violent slasher movies or playing realistic violent video games, so do children become desensitized to that which freaks them out.
Parents do have to be careful about using this approach, especially with sights and sounds that exist to protect them. A child shouldn’t be desensitized to the sound of a traffic light beeping a warning or flashing lights on a stop sign. Desensitization should be used with common sense.