get a buddy/sibling/gf/etc to ride along with you, get the noise going, then have them to touch stuff until the sound stops. if you find out its a panel, go get some 3M sound deadening tape and put it on the inside of the panel.
I had one of these once, guy brought in his year old mustang complaining about a "tinny sound from doors when radio was on" I taped both his door panels, driver's side noise went away, passenger side stayed. Saw his wife's sun glasses in the door pocket, took them out, noise stopped. Told the adviser that the glasses were making noise and needed to be kept in the center console where I had put them. Adviser forgets to tell the guy, he comes, picks up the car, sees the glasses and says "WTF that's not where they belong" puts em back, turns the car on and comes right back in, all pissed off, to complain that the sound is still there. I go out with him, hear it, pull the glasses out of the door, and he just cracks up laughing.
Moral of the story: Check for sunglasses.
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