There's been several threads about this in the past - but it's very likely the shape of the trailing edge of your foils. If they are very symetrical, they tend to oscillate quickly creating the hum. You want to chamfer one side of one trailing edge slightly and this will set up a very tiny imbalance/vortex that keeps the foil from vibrating. You can accomplish this with by scraping a pair of scissors up the trailing edge of the board. Sometimes you can accomplish the same thing by a tiny chamfer on both sides.