The sewing machine sound is not uncommon but not normal. Most common cause for the noise you can't get rid of is worn rocker arm tips. The VW engines rarely get regular valve adjustments and it puts a wear pattern on the the tips and the feeler gauge can't compensate. Did you use the 180 degree rotation when you ran the valves or at least have both valve covers off to double check the back side of the cam for exact placement of the cam lobes?