OK... first off you really have to decide which of your kinds of shooting you will honestly be doing more and why... and then on to your answer:

10D=Quiet Shutter... almost sounds buttery...
Rebel=About the same as the 10D
RebelXT=Midrange...not too loud
1DM1=Midrange... has crisp sound but not super loud...
20D=Loud... loudest of all cameras I have owned....
1DM2=Midrange but smooth

I havent shot the 5D yet... but its in my sights as a camera I may want to get...

of course for weddings you will love the high res of the 5D...but my personal opinion is that the 1DM2 (or "n") is a better purchase for the same cash... the 1DM2 will typically last you MUCH longer for shutter actuations, it has a much faster AF engine, and it typically is better for sports... these are all my opinions... I also look at that how many times will you blow up an image to where the difference in MP makes that much of a difference...

Now with that said and done... everyone always promotes what they personally use... and I am no exception I guess... but I have used a variety of the canon cameras and have kept most of them... I would say comparing the 20D to the 1DM2 for noise its hands down that the 1DM2 is MUCH quieter...

I would say that if you are in an area that you can rent both for a day it would be a great thing to do... or find someone in the area that is willing to let you pop off a couple shots with both... they are both 100% opposite kinds of cameras... so it will really boil down to a lot of personal opinions on them...

__________________
TRJ Photography
National Press Photographers Assn. Member (NPPA)
Canon User:1DMII/20D/10D(IR)/1nRS/Pro-1/Some L Glass