Four days ago I purchased a 2010 Audi A4 Quattro as a CPO. Just tonight while in a drive threw sitting next to a wall with the drivers side window open and the vehicle idling... I noticed a slight knocking noise coming from either the engine compartment or the undercarriage of the vehicle. The best way to describe it is it sounds a though I'm driving the worlds quietist diesel car. I can not hear this noise if I'm just sitting in the car with all the windows closed at a red light or any other situation. Only while next to something the noise could reflect off...then of I can hear it. I'm coming from years of driving Subaru's which have dozens of different knocking sounds that are totally normal but I didn't expect to hear a nose like this from such a quiet vehicle as this Audi. ANY advice would be awesome! Thanks so much!