This is another topic that is pretty straightforward. The deciding factors when considering audible noise are four-fold:
First, how much audible noise each projector makes.
Second, where your projector is placed, relative to your seat. Obviously audible noise will be more of a factor if you've ceiling mounted your projector 4.5 feet directly above your head, as compared to, say, on a shelf in the back of your room, 6 feet up from your head and 6 feet behind you.
Third, whether you are running on a projector's low, or high lamp modes. Low lamp mode is normally dramatically quieter. Many of the noisiest projectors are still quieter in low lamp mode, than many of the quieter ones are, in high lamp mode. I should mention high altitude settings. If you are living in Vail Colorado, or even in Denver, and maybe Albuquerque, you may have to run your projector in high altitude mode to properly cool it. High altitude modes tend to be very noisy. I can't help you there, however, as I do not observe projectors in that mode. Sorry, if you are up high, you'll have to sort that one out for yourselves.