Hard to say without knowing what it is that's bugging them.
But the NAS website does have a page of factsheets about this: https://www.autism.org.uk/about/behaviour.aspx , see especially the last three. Pay attention to the text at the top of the page: "There will generally be a reason for this: it can be an attempt to communicate, or a way of coping with a particular situation. Knowing what causes it can help you to develop ways of dealing with it".
