you could get a rheostat, but that may be a little out of your league from the sounds of your post. Sidewinder sells a pretty simple rheostat if you want to use of one. What it does control the amps going to your fan so if you turn the switch it will slow it down.
The best way to quiet the ole Delta down is with a baseball bat. If that doesn't strike your fancy then you might get consider using an 80mm fan...there are several references on how to do that for GW without getting a special adapter.