That wouldn't bother me one bit after 9000 rounds. I expect better from Sig when brand new. It's not a Taurus.Mine has some rattle but then again its rapidly approaching 9,000 rds......
Both of mine (when new) were typical SIG frame to slide fit. Remember that these are production grade guns with machining tolerances. Its quite possible to get a slide at the high end and a frame at the low end causing larger than normal rail gap.
None that I have owned have rattled either until my latest Legion.My 225A1 has a nice fit. I can honestly say, I've never held a Sig that rattled.