My RX 350 is 5 months old, 8000 miles. It has developed very annoying rattling noises under the rear seat and, sometimes, rear door panels. Any suggestion ?

Are you sure it's not the rear cargo cover. That seems to be a fairly well known issue.

I have had the same issues with rattling in the back cargo area....checked everything, can't find what it is. I have noticed, it only does it at higher speeds.

Do you have roof rack cross bars? If so they could be it since they tend to make noise once you start driving at higher speeds even though the rattling might sound like its coming from under the chair or doors.

I do, I will have to check....it is really starting to drive me crazy.....

Try taking those off and then driving with it and see if the problem still persists. It should stop rattling once you have them removed. Hope this helps.

Check the seat backs are latched in properly as I think they do have the ability to recline a bit and that the rear seat center armrest console cover and cup holder are secure.

If you cannot find anything , it is time to turn up the radio

I had a rattle and it was the rear center console latch not being engaged all the way.