IBM Model M's are loud. Most of the people who type on them like that, but it can drive other people in the office batty. One of the things you can do about this is a floss mod.
Yes, it's just like it sounds, you put floss in the keyboard. Specifically, you insert Super-floss into each spring. This dampens the high-end 'ping' when the springs buckle and bounce back. You can do the same thing with silicone grease (don't use petroleum-based greases, they'll eat the plastic) but the floss mod is less messy, and easily reversible if you don't like it.
I haven't performed this mod on any of my M's yet, but I want to. I'll let you know you it goes.