You could even consider just painting the underneath portion of the sill (only). This would not be visible but it would still reduce noise. I find the sills make quite a bit of noise when dirt is kicked up from the font wheels and resonates when it hits the underside of the front sills. That said, I guess one could also opt for front mud flaps for the same effect. 
There are also internal frame sprays that might have the same effect from the inside. Not sure, never tried this. There are access holes for spraying into the front sills in the front wheel wells behind the plastic wheel guard. For example:
Eastwood Internal Frame Anti-Rust Coating, 14 oz. Spray
 

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