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.
Fourth, and finally, your tolerance for even the slightest fan noise. This is perhaps the biggest factor. There are those who are unhappy with any noise that can be heard during a quiet scene, no matter how low. Nevermind that their hot air heating (or air conditioning) may be several times louder when it kicks on, they hate noise. Most of us are just not going to have a problem with projectors that claim 30db or less, as most do, today. (The loudest tend to claim 33 - 34 db). The vast majority of us are more than willing to live with a 30 db sound level (claims, not measured), and would be thrilled with 25 db or less.
The quietest of today's projectors run a virtually silent 19 - 23 db with their lamp power on high, and lower still in low power mode. Others are more in the 21/28 (low/high power) range, others with a range of 23/30 and the nosiest, roughly 26/33.
For this section projectors are ranked as:
Ultra-Quiet projectors (still very quiet at full power)
Quiet projectors (still reasonably quiet at full power)
Moderate noise level projectors (borderline at high power those who are very audible noise adverse)
Significant noise level projectors (those over 30db at full power)
I am not going by the claims of the manufacturers, but my general experience with each of these, so it doesn't really matter what the specs are.
When I watch projectors in my large room the projector sits about four feet behind my captain's chair. In the testing room, I'm moving around a lot but am typically 2-4 feet from the projectors and can hear all but the most silent.
Other types of audible projector noise
One last thought before the listings: Not all noise is fan based. DLP projectors have the noise of their spinning color wheels, but at least I'm happy to report that the higher pitched whine of the wheels in older projectors are essentially absent with today's projectors. There is one other factor, which relates to projectors with dynamic irises (most of the projectors in this review). Only in a few cases is the iris action audible. Be warned however, that when it is... The older Optoma designs (includes the HD806 and HD8000-LV) have dynamic irises so noisy, that I don't consider those projectors to be usable with the dynamic iris engaged, we're talking a LOUD clicking sound. Epson has a much lessor problem. Some report their iris to be loud enough to be noticeable/annoying. I've factored that iris into the Epson rankings. One thing that may help with the Epson - if you are shelf mounting, put some sound absorbing material between the feet and the shelf, as I find that the shelf (or tabletop?) may amplify that sound by resonating with it. I don't personally find the Epson iris to be noticeable enough to be an issue, but if you are particularly audible noise critical, it may be.