I have found it to be as loud as any other ventrui style skimmer. The noise is caused by the air being injected at the impellers and being mixed with the water. I personally found it to become a little quiter as the skimmer gets some age on it (after a week or two).
You can use it on your sump in a HOB
fashon. It should not be put IN the sump as I dont think the PH is sealed.
For the price I think they are one of the best skimmers you can find. WHen you compair them to a skimmer thats over twice its cost then no they are not as good.
I ran this skimmer in my bedroom with no problems. Then I moved and ran the tank in my dinning room with no problems. Now its down on my 80 in my sump. I use it sparingly.
If you have a DIY canopy it can help quite it down some because the noise has to go thru the caonpy to get into the room.