Looks like I am getting this same noise from left rear. I will test it the way you did. In my case, apart from the lock, there seems to be some damage to the plastic bush where the seat is folded. After last service, post wash and clean, while pushing the rear seats back to position, they left the rear seat belt lock under the seat. So the seat was not coming to the correct position. With almost an inch thick object under it, the seat will never come to position. They probably applied force, breaking the plastic bush! These are small things we are unlikely notice while taking delivery after service. But they spoil the service experience.