Query on Evaluating my legal options against a defective manufacturing

Hi , I purchased a mobile (One Plus 6 ) from Amazon.in on 21st May,2018. Post 2 months of usage (in July mid) the vibrator of the device started making a rattling sound and the One Plus Service centre replaced my handset when I resisted on getting it repaired. Now a similar problem has occurred again and the handset is again making a rattling noise , I read about this online and found many consumers are facing similar problem worldwide (https://forums.oneplus.com/threads/oneplus-6-vibrator-rattle.861017/). It seems the device has a serious manufacturing defect and they are shying away from acknowledging this issue. I went to the service centre again and they have agreed to repair the device , but I'm not okay and I'm pushing for a replacement as this device is again 3 months old. My larger question is that "How can phone devices worth Rs.40,000 start having such serious problems in 2-3 months " Want to understand my legal options in this case and further action