I've got two 629s, one Classic and one Mountain Backpacker (or similar name). They both make the noise. One night it was bugging me, and I removed and replaced the grips about six times, since the only free-moving part I could see was at the base or bottom of the grip butt. It didn't help. So Sam, thanks for the tip, I'll try it this weekend.
For what it's worth, the Classic (5") is the most accurate pistol I own, the Backpacker (or whatever it is, 3") isn't. Pachmayr grips helped the little guy's accuracy, but it's still not where it should be. May just be me.