I did coat at the circuit board with a conformal coating. I used a CRC polyurethane-based. The silicone was stupid expensive.With the circuit board, you can coat with rattle can clear coating spray. Another option is to pot/encase it in hot melt glue (or epoxy if you feel the need). There are purpose made conformal coatings, but they typically are expensive for a one off job.
I have wondered if riveting a small spoiler to the top of the fan cover would produce enough downforce to prevent the rattling? Mine does it some, its much better since I moved my light bar a bit down.
The spoiler is an interesting idea.
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