I used my samsung tablet "sound meter" app, which seems pretty accurate, although it is *not* calibrated.
when quiet, my room is about 26 dB. from 2 feet away, the 5010 starts with a fan noise of about 35 dB (ignoring the non-fan noises, like the lens cover), and then settles into eco mode with a fan noise of about 28-30. it is fair to say that the 5010 is quiet in eco mode, but it is absolutely not anywhere close to silent. I can easily hear whether the projector is running or not on from about 12 feet away. of course, during movies, I presumably will not listen for fan noises---but it will be obvious in quiet scenes that it is running, unless it is used as a rear projector.
for comparison, the eco mode level on the 5010 is about the same loudness as a Shuttle XPC home computer. in contrast, the Pioneer Kuro 6020 is practically silent (and unmeasurably different from the background 26dB in my room) from 2 feet away, although it is audible if one puts one's ear on the glass.