All of this could be true, but in my opinion, I really think the cloister bell was part of his fear, which is why I'll stick by my theories of universal destruction for now. Another possibility is that the cloister bell could have been signaling an impending regeneration, like it did with the 4th Doctor. That could certainly make sense since the Doctor has previously not wanted to regenerate...
In any case, this was a great episode, and I'm actually glad they didn't show what was in the Doctor's room. Also, a nice touch was the Doctor calling Amy "Amy Williams" at the end; it was long overdue but a very sweet moment.