There are a couple of things to take note. First of all, figure out is the sound coming from the board behind. Normally on the both side of the board, there is a hard spongy thing for the boot to compress on it and it holds the board. Normally, it is insufficient. Which is why, I have added a sponge using a velcro strap to strap it on both side, and this holds the board down.
2nd, remove your board, take note that your board is held together on both ends by a small latch or a cylindrical assembly. Due to the plastic nature and the point of contact, you may want to wrap it up with some sponge (it doesn't last long) or some sound absorbance pad. This way when you put fix down your board onto the latch, it doesn't vibrate and cause sound.
3rd, after you place your board in, when the board is in place, before you close your boot, just take a look at both left and right. The board should sit nicely within the groove of the back. Well, you can have some padding there as well as original padding is not good.
That settles all issue from the board.
4th, just make sure the assembly at the rear trunk are all proper and in. As the car has aged, it is no suprise that the screw holes are damaged. If you intend to remove the assembly and check, becareful as the brittle plastic screw holes may crack when you try to assemble them together again. Mainly check for any misalignment of assembly as, if any of this part is not installed correctly, it will cause friction and sound.
My advice is, get someone to ride along with you, and he/she will detect where the sound comes from. If you hear it during driving, you can never pinpoint the exact location as the sound bounce off and may assume it comes from other place. I used to do this with my brother.
I dynamat my whole car, basically trunk, doors, roof, and going for tyre compartment (already bought but no time to do). This helps to reduce the sound a lot but you may then get irritated with other very soft sound which you never experience before. It goes on and on.
Good Luck trying
