The company I work for was about to be forced to buy noise reducing headsets due to the noise level on the 737-300 (although the noise levels were very close to the health and safety limits) ...
The irony is that as fuel costs have been going up, our cost index (same as every ones else's I guess) has been coming down. Cost Index now at 20, which means at standard speeds, were are generally within guidelines.
However, if you accelerate a 737 from 250 to 320 knots at say F150, the difference in noise has to be heard to be believed !