noise in head

Hi, my five year old step son has always been hyper active and some what defiant, we will taking him to have a psychiatric evaluation for possible ADHD, but recently ( in the past 4 months) we have caught him hitting him self in the head, when we ask him why he is doing this he tell us that "there is too much noise in my head" we asked him to tell us what the noise sounds like and he let out a very loud scream! we are concerned for him and his mental health, does anyone have any idea what could be going on with him? and does anyone know what kind of tests that might be given at the evaluation? we are very concerned.
Thanks
sam

[LRM1021]

At the evaluation, you will most likely be asked to complete some forms describing your son's behavior as well as some forms on his general medical history. The dr will also meet your son and ask him some questions as well.
I have heard of children with ADHD talking about "noise" in their heads-- their minds are running amile a minute and thoughts are racing around-- this could be perceived as "noise."