Jordan

Hi, I've recently brought an A5 3.0 tdi quattro 2015. When the car is full warmed up after driving for 15/20 minutes the idle speed seems to be dropping down low (600rpm) and generating an engine woddle/shake only at idle, drives fine but just vibrating a lot on idle. I wondered what everyone elses idle speed is once the engine is fully warmed up? I took it to audi and they agreed it needed to be looked at and didnt seem normal but after having it in and running the diagnostics and health check they couldnt find anything and said its probably just a characteristic of the engine but im not too convinced as it seems to be shaking way too much than any another other car i've ever been in. Thanks