I had this ratting sound too when mileage was about 1k km, the rattling sound came from somewhere at the back passenger (that was what I thought) and yes it was worst when it was hot like ycong1969 commented earlier. After complaining to SC during the service, test drive with them for a while, they acknowledged the rattling sound, then it took a few hours to locate the source. It was not from speaker or blinds or seat or battery compartment...but it was from the roof! Take out the inside carpet of the roof and you will see something like 3 crossbars (not sure if I used the right term), front middle and rear. These are metal and rivet to the roof underside. There should be a silicone seal between the metal crossbar and the roof and in my case, the silicone for the rear crossbar was missing...someone forgot to install when he rivet that bar, so the metal crossbar and roof had no silicone in between causing them to knock thus the rattling sound. A simple test you can do to see if this is the same rattling sound as mine is...you seat at the back...and have someone...slowly knock the roof from outside using hand...knock repeatedly...at front middle and rear...and observe got the sound or not...The SC did that in front of me and walllahhh....that was the rattling sound I've been hearing all the time, maybe because its the roof, its worst during hot day. Mine was the 1st such case for the new hybrid...SC had no solution that day, and said they will check with 'bahagian pemasangan body' to see how to solve. The next day, SC called and ask me to leave car for 2 days, it actually took only 1 day and job was done. SC said they install back that silicone. Now clocking near 10k km and the sound is gone. Hope this helps.

I service at Toyota Wing Hin, Pandan Indah. Don't remember the address.

Do you know the T&C. Called my SC this morning, they not sure, must wait for 'manager'...and he is on leave.

I posted before about the roof issue I had, maybe same issue as mine