same here, RX 300 04, have taken everything out to isolate the problem so far
have emptied
all goods, storage
taken out rear shelf cover completely
all tools, jack under boot stuff out of car too
secured spare wheel to most possible
even the little metal thing that holds the under boot storate open
still the same noise, quite annoying on these unmaintained small roads
so would be most glad to hear if anyone finds an answer