I know this from design of silent computers, an area where large fans are popular. :
Air moving fast produces noise. With a large fan you move much air flow slowly, with a small one you move little air fast.
Should apply to jet engines also.
But airbus seems to have better noise insulation, because their planes always seems to be more quiet...
Edit: One important thing when it comes to noise in aircraft is where you are seated.
It's much less noise in front of the engines than behind them.
[Edited 2008-03-20 19:18:27]