RE: Drake slalom Pro 44 whistles ???
Mostly sharp, strong fin whistle comes from the trailing edge finish.
Each fin is slightly different from the (hand finish) in the factory, so without being able to view the fin up close, the general cure can be to VERY LIGHTLY sand the back trailing edge TE of the fin with some fine (400 - 800 Grit) Wet+Dry sandpaper. Best is to use a firm block or similar to support the paper flat across the TE, not allow the paper to "wrap" around the TE as you sand.
Secret is to hold the block on a mild angle, so rather than sand the very fine TE "square" (or flat at 90' to fin centerline), you actually hold the paper/block on a slight angle to 30-40 degrees when viewed vertically looking down the TE. Just a couple of very even but light strokes should solve the situation.
Often if the TE is finished too symetrically, the waterflow "collides" as both sides of the flow come together off the back of TE. By angling the TE slightly, you minutely shorten the flow on one side, which disrupts the harmony of the flow/s, and they actually merge more smoothly, without the whistle, which in turn often can indicate some minor loss of efficency.
Saludos ~ Ian
Paco, can send you a diagram/email if the description is not clear.