No way to say if there are other welds that are marginal...but let BMW speak for their confidence in their 'fix'...

I'd give them two options (if you can):

1. I'll turn the car in immediately. ( I assume you have this right, and will receive a pro-rated refund.)

2. If I keep it and any new noise turns up from the rear of the car, I will receive every penny back that I paid for the car.

They can chose which they'd like.... if they choose #2 it needs to be in writing.

A