do you have a bug deflector? If so, GM has a fix in development for ya, the problem is the air turbulance across the deflector. Temp fix is to put some rtv across the cowl seal area to keep it from flapping.
I have heard another rattle from a pick up like that it was coming from a piece of foam under the black plastic cowl trim.I think the foam is used as a dam for water to not run directly into this area.The customers complaint was at about 60mph and up this noise would occur.Reglued the foam noise was gone.
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