Vibrator !!! Vibrating roof skin that is. Advice needed please.

[Grizz1963]

Hard to believe but my truck can exceed 50mph.

However, at that sort of speed the roof starts vibrating with an irritating drone.

My truck has that padded headliner thats mounted to presumably 8mm plywood.

When you look at the outer roof skin, there is a dent toward the rear but also a hole that seems to have been filled with filler or window putty and painted over.

So I am not sure if someone has previously attempted to silence the drone successfully.

I am NOT removing the roof skin.

So what possible solutions are out there that works.

One idea (may be totally wrong) is to drill 4-5 holes from inside where the headliner sits, and to squirt expanding cavity foam in there.
Another could be a 500ml tube of silicone pumped in from the top or bottom as I want to weld the hole closed after treatment.

Advice, photos, warnings all appreciated.

Thank you.