Hey buddy, lemme give it you straight from a person who eneded up using about 4 different forms of sound deadening. Everything they have on the market will not make a centrifigul, or even inline fan noise dissapear. Inside my fan box, i used Green Glue in between 3/4 inch thick plywood sandwiches, Mass Loaded Vinyl on the oustide of that, and then accoustic foam in all of the empty cavity, and albeit my fan is a bit quieter, its not nearly noticable for the amount of skrill i dropped into it.
My one suggestion for you would be a variac instead. Its basically a voltage regulator you can use as a fan speed controller that wont make it hum.

out.
PurfectStorm