Hi There,
I have experienced exactly the same noises over the past few months but on my 2008 RX 400h.
Needless to say they have been driving me bonkers. I asked my local garage in London to have a look but they could not identify the root-cause. Also the car passed MOT.
Fast forward a few months and the noises are still there and the ride is bumpier that ever. I took it to a local garage in France to fix a load balancing issue on the front tyres and asked them to have a look. The mechanic said that the springs are covered in oil and that both rear shock absorbers need replacing ...
I origanally suspected that some srt of bushing dried out and caused the issue but the mechanic was clear this was not the main reason. Having said that, the car is near 120k miles so changing them would surely help as well.
Let me know how you get on