From the HGTV website: One sone is about equal to a refrigerator running, while four is equivalent to normal conversation.
OK, I'm risking your wrath because what I am going to suggest is not rated in sones....but, FanTech inline exhaust fans allow you to locate the fan itself far from the listening room, so that you don't hear the fan motor at all. You only hear the air moving through the grate. In my BR with the 100 cfm fan on full, you can hear the air quite easily through the 6" grate. At half speed it is totally silent. I'd guess that an oversized grate would make it quieter, perhaps even silent, at the highest setting. Unfortunately on their site FanTech doesn't list sound ratings, because they say that there are no testing standards for remote mounted fans.
OK, I'm risking your wrath because what I am going to suggest is not rated in sones....but, FanTech inline exhaust fans allow you to locate the fan itself far from the listening room, so that you don't hear the fan motor at all. You only hear the air moving through the grate. In my BR with the 100 cfm fan on full, you can hear the air quite easily through the 6" grate. At half speed it is totally silent. I'd guess that an oversized grate would make it quieter, perhaps even silent, at the highest setting. Unfortunately on their site FanTech doesn't list sound ratings, because they say that there are no testing standards for remote mounted fans.