RE: Drake slalom Pro 44 whistles ???
Ian has it right, but I do it slightly differently.
Run the sanding block down the TE of the fin with the block perpendicular (90 deg. or a "T") the first couple of strokes with the sandpaper to create a very tiny flat surface that's 90 deg. (perpendicular) to the front to back axis (and the water flow) of your fin.
Then run the block down each side of this tiny "flat" at a 30-45 deg. angle so you slightly "chamfer" or dull the sharp edges where the surface of the foil meet the tiny flat created in the first 2 strokes of the sandpaper.
Also, get a straight edge or dowel pin and run/roll (the dowel pin) down the TE of the foil and look for any little "hollows" just in front of the TE. The hollows create "thinned out' area that can whistle as well.
To correct the hollows, sand the foil in these areas (blending in with the surrounding area through the use of a sanding block) until the hollow areas disappear.
Then break the sharp edge as Ian suggests, and put the tiny chamfers on.
As far as causing directional problems, you aren't going to be taking off enough material to cause any sort of "asymetry" so there should be no worries.
Also, even is you are correcting for "hollows" you are going to be working on the very last section of the foil, right at the trailing edge, so you would only really change < 1% of the actual foil.
Very fine wet/dry sand paper only removes < .0005" (0.0127 mm) per stroke.
Hope this helps,