For what it is worth, I have two 4 cfm Walter Kidde's running in parallel, electric powered.
With a long snorkel running outside my garage and a silencer on the end none of my neighbours have ever complained.
It makes all the difference.
I just asked them tonight and confirmed that noise has never been a problem in our suburban neighbourhood.
I run the bloody things at 11 at night and no one has ever batted an eyelid.
Mind you, when I bought the first one I had to try it even though it was gas/petrol powered.
It was only 9 at night, the cops turned up at 9.45.