Noiseblocker BlackSilentPRO PK-2, 140mm
Wow, dead quiet outside the case, but noisy inside... Even as a front intake it was noisier than the Alpenfohn. I have no clue why. Looked lovely but as an intake it give a vibration sort of whine. As an extra front intake it wooshed too loud. Maybe it moved too much air? Also, the rubber square frame to fit on is a great idea but ended up as a big frustration. One of the most annoying assemblies in my computer life ever.
I'm using a PK-3 (the high-speed version) as a top exhaust, so I would like to add my own two cents.
First, while installing the included gasket is a little annoying, I found it to be well worth it. Not only is it a highly effective decoupler, it significantly increases effective airflow.
Second, at its lowest speed, my PK-3 is quieter than a Nexus 120mm but moves a whole lot more air, and even at full speed that moving air is pretty much the sole source of the noise. I haven't noticed anything remotely resembling whine or vibration. However, all that air can make a racket if it's forced to go through too small a space, so restriction can be a problem.
Frankly, I'm surprised that you felt so negatively about the PK-2, which should be even quieter. IMO, the PK-3 is a good choice for mid- to high-power systems. You have to keep it on its lowest speed, but that shouldn't be a problem, as the only reason to turn it up is if you want to see your case take flight.