C does this too.
With the cat.
It only ever seems to be when he isn't doing anything, which is most of the time when you're a cat.
I THINK she does it for a reaction, but when he DOES react it's painful for her as he will scratch her. Nothing I can say will stop it either. I guess it's maybe down to the fact that she (like a lot of Autistic kids) doesn't understand cause and effect. No matter how many times it happen, she's always surprised when after screaming in his ears he attacks her.
I can now no longer leave them in the same room as she can be a right pest and he has a short fuse. I have noticed though that she will start that horrible giggling while she is tormenting him, so even if my back is turned I can either remove him or distract her.
I have repeatedly told her that he is asleep (when he is, obviously!) and he doesn't like to be woken up. She must let him sleep as he is a bit old and a bit sick and he'll wake up grumpy. When he's awake and day dreaming I tell her that he will be scared and how horrible it would be for him.
I have no idea if it works, as she will repeatedly pester him, but I feel I have to say something to her in the hope that one day she WILL understand.
Misty, can DD sit in the front of the car? Or can the baby go in the front instead?
If she absolutely must go near the baby when he's asleep, can she be encouraged to make a quieter noise? maybe a little quiet singing or a whispered nursery rhyme? It might be less scary for him.