Let them buy the car back mate I recon...
We recently had to buy a 6 month old hybrid volvo back because the main battery kept draining, and the car wouldn't start, we had a 30 year experienced volvo master tech spend 40 hours on it, volvo hq collected the car 2 weeks ago and still don't know whats wrong with it.
I know its a completely different and probably more difficult fault to trace than a noise, but cars are too complicated now, and certain things can be really hard to trace
Hope you get something sorted soon !