put the fan in a box fill the box with sound insulation or rockwool works and use acoustic ducting this will reduce the sound in the grow room significantly
although this will not reduce the sound at the outlet, if you also need to remove this sound you will need some kind of muffler which is better to avoid as it will reduce the flow somewhat,
i always vent from room to room never directly outside
the outlet noise is not loud enough to be heard by neighbors overall it sounds quite natural like a gas heater / water boiler
[video=youtube;WKk8h05GjYw]http://www.youtube.com/watch?v=WKk8h05GjYw[/video]
this shows rockwool works pretty well, although this type of high pitched noise is
easier to block, with the fan you will also have vibrations, bubble wrap works well to remove vibrations
if suspending the fan from cords is not an option
peace