Hmm interesting, I have a bit of a rattle noise going over speed bumps myself. I wonder if it's the end links though.
Keep us updated, as I'm on RSR Down's too. Might need an aftermarket solution if the Lexus ones aren't going to hold out.
2014 IS350 RWD F-Sport
All-Fit Front Lip Kit || RSR Down Springs || 20 MM Spacers || VR Tuned Flash