73:  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 119.