Two questions; Firstly all my mileage is around.

Two questions; Firstly all my mileage is around town, mainly low speeds. I have bought from a dealer a lovely condition Audi A8 TDI SE Quattro. Every so often I hear from the engine compartment when driving something like a compressor or other engine running, quite loud, with some vibrations for about 30 seconds then it stops. By the time I stop to open the bonnet it has stopped. This does not happen when left running stationary. Secondly sometimes when I stop the car ` vents ` deisel smoke grey from under ther front offside underneath of the car.I can see and hear the car settling as it has the 4 stage variable suspension system, what could be the noise and why does it vent diesel? This doesn not happen every journey?