most trunk lids have 2 layers there the actual trunk itself and then has somekind of brace that is welded to the underside of it. my buddy had some serious sounds in his trunk and layered it like hell and foamed it. and still rattled like crazy. he ended up taking all the mat off the lid and spot welding the 2 layers in a few inch increments and then layering it again with dynomat and had the trunk lid repainted and made a hell of a difference. but he was a welder and sprayed his own lid. but thats bout the only way to really calm it down without sealing off the trunk.
Also try to deaden or insulate the areas that the lid contacts the trunk opening. Sometimes you can replace the rubber feet if they are missing. Most older cars have adjustable trunk lid feet that have rubber cups on them. Try adjusting the trunk lid up a little ways so it is not banging metal on metal. Replace the seal if it is missing.
back in my old car, my friend noticed that if he pushed down on the trunk it reduced the rattling a lot, so the latch must have been worn out, maybe that's a common problem
I applied almost 4 layers through out the trunk, lid got 4. Still rattled. I listened to it for a min and found that my third brake light in my spoiler was the culprit. The board in there was shaking in side the plastic. There was really any way to get to it. So I took a fine drill bit and drove it in between the board and plastic. Filled the hole with some sealant now almost all gone. My sunroof is the issue now. Every time you fix something, there will be another noise posit you off more.
Don't pressurize the trunk... only real way to get rid of it.
I am pretty sick of it myself in my CTS so I am building a 4th order with the port coming up through the rear deck where the factory sub use to be. **** sure shouldn't have any trunk rattle after that! LOL
It's a 2012 Buick Regal and I have lowered the latch 1", licenses plate is good, lights are good. I did find that when my friend seat on the trunk lid it stopped, now if I could just keep him there I'll be good.. Lol
Anymore advice please? The Buick symbol is about to fall off.