69:  Why does Emacs spontaneously start displaying `I-search:' and beeping?
  
  Your terminal (or something between your terminal and the computer) is
  sending C-s and C-q for flow control, and Emacs is receiving these
  characters and interpreting them as commands.  (The C-s character normally
  invokes the isearch-forward command.)  For possible solutions, see
  question 133.