cheap fans have cheap blades that cavitate (vibrate) due to differences in air pressure at the tip of the fan blade and the base of the van blade.
this happens because the fan pushes a relativly large amount of air through a small opening, which compresses the air as it moves across the fan blade.
in plain english, it means the fan generates alot of backpressure...
throw in the squirrel cage motor that whines like a jet turbine, and u got a noisy sunuva bitch
there's not much you can do to quiet it down.... but you can mask it pretty good.
if you mount the fan to a board, and suspend the board with bungee cords (to deaden any vibration/harmonics) it will help.
and then wrap whatever is still exposed in the highest r-value insulation you can afford, and that will muffle the rest down to an acceptable level.
i dont use duct booster fans just for this reason.
but i can get away with it, stealth is not a major concern for me.
edit/ps-
as far as where the fan is.... they work best pulling air from a room than pushing air out of it....
the harder the fan has to work, the more the blades cavitate, and the louder and more inefficient it gets.
and another note- rigid ducts (like the kind that do not move or flex) can actually amplify sound... think of the old voice pipes they use to use on ships, sound travels through tubes pretty well, so if you have rigid ducts on your system, get rid of em and get flexducts... if you notice your house's a/c system is run on flexducts for the exact same reason.
