The pic of Pennywise above gave me the idea :-)
Hope your little brother isn't real little, if he is, I would feel bad about us giving him nightmares.
Real answer.. if he lives in the basement, and it is in an inner wall... could be water, heat or something like that in inner walls, house settling, a rat, possum or some other critter, could be a snake.
If is the outer walls... I have no clue... something might be trying to burrow in. Do you have badgers there?
I assume the outer walls are concrete... badgers would have a tough time getting through.
Don't even get me started on "Tremors" creatures coming through the walls! LOL