Pathetic after service

I purchased a brand new VW Polo Vivo 1.4 trendline, on 11th October 2017, since 10th November 2017, the engine has been making funny noises sounding like diesel car.

I went to VW on Monday 13th November 2017, and explained to the mechanic from VW what the problem is, i also played a voice recording of how my car was sounding to the mechanic and told him the noise comes and goes, it's not consistent, that's why i made the recording. They booked my car for a diagnostic on Thursday 16th Nov 2017, and when i collected my car, the mechanic from VW told me that he drove my car and could not hear any noise. They said they gonna book the car for diagnostic which never happens and i've been calling the managment and they are not helpful telling me there is nothing they can do to assist me and refused to do a diagnostic to my car which will tell what the problem might be not driving it. I was adviced by the mechanic and the managment of VW that when the noise comes back i must call them but i wonder how this will work if the noise happens while im out of town.