The Sol Cat 18 and 15 were not foam core boats. At the time they were built the most advanced hulls were on the Stiletto 27 with the Nomex honeycomb, aircraft epoxy, and E-glass.

Try to determine if some of the cracking sound is coming from the under the deck, outboard, where the top of the hull meets the underside of the deck. Look underneath just inside the lip of the deck to see if you see any cracking there.