hi that sounds like the focus screen – take it straight back to be fixed and rest of camera should be checked as well.
@j.J. 152200 wrote:
I was busy taking pictures of a butterfly with my new 5D mark 11 this afternoon when suddenly a framelike thing appears in the view. I removed the lens…for the first time ever… and I saw something dropped from the camera. After careful investigation I found that the frame that hold a little “sandblasted like” square glass of the viewfinder in place, clipped lose. I carefully slipped the “glass” back into it’s frame and put the lense back on…just to found that there is now a scratch on that piece of glass and the scratch is very visible through the viewfinder. And to worsen this bad experience a few specks of dust also got in while I was fixing this problem.