I used to be able to tell which of the long row of Model 33 TTY's was left on at the college computer center by whistling the 300 baud carrier and modulating it

I used to be able to tell which of the long row of Model 33 TTY's was left on at the college computer center by whistling the 300 baud carrier and modulating it a little. The ones that were on would start typing gibberish.