Clay, foam, or anything gummy in direct contact between the vibrating surfaces will generally stop audible vibrations, but the location is the key. Lithium or other thick greases work great if it's low mass, meaning not a lot of material is physically moving.
If you fill the wing with foam it will work if you dampen the interface, but if not you'll have a foam filled wing that still makes noise. If you can reproduce the noise with the car parked, get a makeshift stethoscope and try to pinpoint the problem before you decide on a good solution.
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