Just an update. All 4(!) of the G403 wireless in stock at my local Best Buy had lens rattle to some degree. I ended up keeping the one with the least audible rattle. You can imagine the stares I got from customer service at my buying and returning so many of the same product.
FWIW, the G900 and G502 on display had no rattle, but the G403 wired did (they didn't have the wireless G403 on display). Ofc those have likely been on the shelf forever and abused, but thought that was worth mentioning. I think either that Best Buy got a bad batch, or the G403 is just an inherently cheap product (sounds funny calling a $100 mouse "cheap") with questionable craftsmanship and quality control. Also of note, in addition to the inconsistent rattle, was the inconsistent fit and finish of the 4 boxes of G403's I opened. Some had very misaligned and/or scratched Teflon feet out of the box, some had small protrusions between the plastic and rubber panels that should've been shaved smooth at the factory. Just somewhat disappointing for a mouse that now costs 2x of my previous one, the G602, which has much nicer build quality (albeit significantly heavier with vastly inferior sensor and wireless tech).
Edited by octiceps - 5/1/17 at 9:17pm