hi im hoping somone can help iv been to couple of garages and they dont know what it is

when i start my car up and its ok but if i leave it idleing for around 3-4 min it sounds like there is a woodpecker under my bonet if i tap the accelerater peddel it goes away then after few min it does it again car drives spot on but if somthin is on its way out id rather get it sorted before i goes completly

by the way car is 05 a3 tdi dsg

thanks for any help