Mason has just started doing this! I know he is quite a bit older but it could be a similar reason behind it.

I aked him why he is doing it and he said he doesn't know but then he said that when he talks like that the other kids can hear him. Not sure what that means but I think it means he repeats a word or letter till he knows he has someones attention.

He almost completely stopped while we were on holidays and yesterday afternoon when he came home from school he was doing it again quite badly.

Do you think he could be mimicking someone? I'm going to have a word with Masons teacher and find out if she thinks it could be a mimicking thing.