I just upgraded the fan(s) on my antminer so I decided to post the details of my experimenting.
I purchased 2x
http://www.newegg.com/Product/Product.aspx?Item=N82E16835103192 from newegg.
The specs on the fans: 95CFM @36dBA @2000RPM, ??CFM @28dBA @1600RPM, 52CFM @19dBA @1200RPM.
First, I tried adding the second fan as is. All it would do is try to spin and stop. It never stayed spinning for more than a 3-5 seconds. This was because the PWM is based off the main fan, which has a higher range of RPM, so my second fan wasn't getting enough volts.
Next I tried using the 1600RPM "Silent Mode Adapter" that came with the fan. This worked, and lowered my temps from 49/47 to 46/43. My primary fan also lowered from 2280 to 1980 RPM as a result of lower temps.
Then I tried replacing the original fan with the second fan I got from newegg. I did not use the RPM adapters, and let PWM do it's thing. Fan speed was slow, only 700-900 RPM, and temps rose above 52 in a few minutes, so I aborted this test.
Next, I used the 1200RPM adapter on both fans. Fan speed reported ~1200RPM, and temps were 52/50 after 30 min.
Lastly, I used the 1600RPM adapter on both fans. Fan speed reporting 1800RPM with temps of 45/43 after 30 mins.
Conclusion: with 2 of these fans using the supplied 1600RPM adapters, I dropped my temps by ~5C, and noise level by ~10db (guessing). Now my antminer is not any louder than my computer.
ps. These fans could run faster if, instead of using the 1600RPM adapter, I remove the 4th pin from the fan power connector. This would make the fan(s) always run at full speed (2000RPM). I did not try this since my temps/noise are great using the 1600RPM adapter that came with the fans.