Doug,
Exam proctors coming into the testing room like you describe would be a clear signal for me to get up, go outside and complain to them right there on the spot. This sounds rather unacceptable.
Students must be aware that there will be some noise and some disruption during the 4 hours that they are in the testing room, but "two or three times every minute" is at an unheard level. I would like to suggest that it might be a good idea for you to write a short feedback to Prometric and send the email CC to PMI for their information.
And to everyone else: if the distractions are at the level that Doug describes you should raise your voice instantly.