I noticed this prob a cpl months back b4 cold snap and was only on drivers side , checked the plenum chamber no blockage , I been told its door seal on the driver side especially if your door has dropped a tad , all vw audis ive had in past suffer from same prob whats needed is new door hinge/s and possible drivers door seal , on the other hand could just be excess water from yer feet during this weather , truth be told not much to worry about if it does it next summer then fix it