I rang Garmin support as I wasn't sure of the warranty details. It turns out I'm 3 months out of warranty, and they won't repair it, but offered to sell me a factory refurbished model for ~$196, with 3 months warranty.
I got around to opening it up this morning and immediately saw the cause of the rattle - the large rectangular blob at the top of image 1 was floating around inside the case. Given its relative size and weight, it was held on by a ridiculously small amount of solder, and not surprising that it came loose.

where it was

where it is
I might have to try my hand at fine soldering and see if I can get it reattached. Pushing it on v hard while outside, while the unit said "Locating Satellites" appeared to have little impact.
Can anyone confirm that Garmin items bought from the UK will be repaired under warranty in Australia? I've heard stories either way. Given the relative difference in price, I'd probably prefer to spend the little bit extra and get the extra 9 months of warranty