FWIW, I've only ever had mudguards rattle twice:
1) Pot-metal mudguard on BSO, having suffered a structural cheese failure. Bodged with cable-ties, lasted the remainder of the life of the bike (a total of about 800 miles).
2) Perfectly good SKS Chromoplastic mudguard - that had given flawless service for about 21,000 miles - jettisoned a stay bolt and started making horrible buzzing noises after a couple of hours exposure to Dutch cyclepaths. Fixed at the roadside with my emergency backup M6 nylock, no further issues.
Make of that what you will, but I reckon rattling is a non-problem to a competent fettler, especially if there's some sort of quick-release thing going on and causing them to get regular inspection. More common mudguard issues are full guards getting clogged with mud[1], and stays getting bent while parking the bike, causing them to rub on the tyre.
[1] See previous comment about how quickly the comedy of off-roading on a Brompton wears thin.