Grease could ball up if there is a change in contact angle if the surface chemistry of the metal changes on exposure to salt water. Precipitation of a deposit is another possibility for example a Calcium Carbonate or another Calcium salt. It may be that there is a salt left over from anodization bath that precipitates the first time it finds sea water. It would be hard to every know without somebody analyzing the specimen.
In any case, many users have reported the phenomenon and have all agreed it disappears promptly.
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From the Ketos Foil FAQ discussing depostis on the carbon:
https://ketos-foil.com/en/content/11-faq-ketos
My foil whistles or vibrates
A brand new foil may whistle or vibrate if not assembled correctly.
Check that you are using the right screws in the right place. Remember the stabilizer has 2 screw sizes, they are pre-mounted when you receive it but check that the shortest is in the rear, the longer in the front. Verify stabilizer alignment.
What if it still whistles? All of our foils are handcrafted. It may happen that a very slight defect escapes us. We invite you to fully mount your foil and check the leading and trailing edges, that there is no roughness, resin droplet, ect. Please contact us if you detect this
I've been foiling for a while and recently its started to whistle.
Check that there is no impact damage. If so, do not panic, it can be repaired. Close your eyes and run your hand over your mast, is it rough? Limestone and micro algae (if you are sailing in a lake) or salt and micro algae (if you are sailing in the sea) have settled and crystallized, creating micro reliefs. Rinse then with fresh water and if that persists wet sand with 1500g and get in the habit of rinsing and wiping your foil regularly. Check the condition of your front and rear inserts, the surfaces between parts, insure there is no play or impurities possibly causing a poor connection.