Where is the Mac 25 cored? "Crackling" sound in the cabin benches?

[HeelBoy]

Well, the decks and soles and cockpit etc were all great, It was stored in a pole barn out of the elements, so I buy the boat.
I get it home, gutted it completely last night, and started cleaning.
I plop my butt down on the port settee, now without mattress... "Crackle".

Do these have a plywood core to delaminate from? How would these delaminate but not the exposed deck etc?

 

[THECUSCUS]

There is plywood core in the top deck, basically where the non-skid is (the blue parts). There is no coring in the inside liner. It is a lightly built boat and will creak and groan when walking about. Not to worry, fiberglass is pretty tough stuff.

 

[THECUSCUS]

This is a cutout from the deck, near the mast.