I would guess resonance to be the cause - as you say the fan is relatively quiet on it own but when screwed into the unit you get a noise. I can imagine the panels of the outer cover being made from very thin sheet metal and vibrating in sympathy with the fan. Perhaps if you glued some damping material to the larger, inside panels you might be able to reduce the noise.
Peccavi
May 2009
howtomendit.com for free repair help, information and advice.
