Product review (1 stars): WISH WE HAD BOUGHT IT HERE, SO WE COULD RETURN IT
We bought a Broan E6448TSS with dual centrifugal fans. Yes, as another reviewer stated, it sounds just like a helicopter taking off-- but next to a 150000-volt electrical transmission tower. The ELECTRICAL HUM NOISE is HUGE, and vibrates our entire house, NO MATTER WHAT SPEED is set. We thought it was the controller; but by isolating the fans, determined it is the fan motors.  When we called Broan Technical Support, and said our brand-new E6448 was extremely noisy, the lady immediately responded:  "It's a VERY noisy unit!"

Bottom line:
It looks nice, and has:
Variable-speed fans,
Automatic heat sensor,
Three lamp receptacles (with difficult to insert lamps)...
BUT
We are not going to use our brand-new kitchen until we FIND A REPLACEMENT FOR THIS BRAND-NEW HUNK OF JUNK my wife unfortunately bought (before the first review).

UPDATE (7/'14): We solved our problem by adapting our hood to control an external blower mounted near the roof -- see 4-star review at Broan Broan 336 Exterior Blower, 1500-CFM, 3.8-Amps, 120-Volt (Tools & Home Improvement).