Was sort of wondering about the same thing on a AT102W. Too be honest, I didn't enjoy the switch feeling too much even after ghetto-click-modding it (rather uneven between keys and a lot of friction in general - might try to teflon-coat the moving parts in the future), so I just let it sit idly in my cupboard.
I guess the problem is caused by the metal stabilizer bars having a bit of room to wiggle around in their guidance "rail" (I mean the litte bit of white plastic inside the space bar key). On my board, it's only the right side stabilizer that seems to rattle, as proven by gently shaking either sides of the space bar. Now, I think it might be possible to twist (and thereby put pre-tension on) the stabilizer to a point where it stays in contact with the guidance rail all the time on both sides. Otherwise a bit of lithium grease applied to the guidance rails could fix the problem as well, by dampening the movement of the stabilizer a bit. In fact, you'd probably want to try the grease method first, as it's a lot easier to remove than having to straighten out bits of metal wire.
Try it for a while though before calling it solved, if the stabilizer moves the grease out of the way bit by bit, you'll have the same rattle after a few hours of typing. Since I don't like my board so much in the first place and don't have too much to lose, I'll see if the twisting method does actually help.
Chris