Can you provide video evidence to your dealer? Luckily (or unluckily), ours makes this noise constantly - both in reverse and forward now, so the dealer themselves could easily document it with video, and we submitted a Dealer Product Report. However, that doesn't seem to do much, as Toyota (in my case, in Australia) has to acknowledge that this is, in fact, a problem. Which they are still 'investigating'. I'm taking regular video footage of the issue for documentation purposes. I've contacted Toyota Australia with a customer service complaint. Auto email says I will receive a reply within two business days, and ....nothing so far.
I feel we all need to put significant pressure on Toyota directly - publicly (not the dealers) to acknowledge this. Comments here are great, but would draw more attention to the brand with comments on YouTube videos, Facebook, etc...