I've recently came across dell enviromental info. (look at my posting in this forum) It's said 630 is the second quitest of all dell notebooks in 14' and up for screen size category. First is latitude d610 with a very small difference.
Are you sure you've read it correctly? The documentation states the following:
D610- 30.1 decibels (idle), 31 decibels (HD seek)
630M- 24.2 decibels (idle), 25.8 decibels (HD seek)
The 630M is clearly quieter. (it is the Sound Pressure Operator Position that you should actually be comparing with, as this is the level of sound that you will typically "experience")
would support that finding, as it mentions that the D610's fan is always on, which is unfavourable for noise.
D610 Environmental Spec
630M Env. Spec