it seems too fine to be a hair... probably dust.. i had the same problem with my 350D earlier, and it is dust trapped under the focus screen, no matter how i blow or wipe the surface of the focus screen, the dust will remain there, then i took the courage to remove my focus screen to clean it..

refer to this pdf on how to remove the screen, it was meant for 20D, but i found it to be the same for my 350D, and should be the same for ur 400D too..
http://www.katzeyeoptics.com/files/KERR-20D.PDF

do it at your own risk. if u're not confident, then send it to a shop and ask them to do it for u..