Fiberglass is not the ideal material for shower receptors and tubs. Today high quality acrylic units are the preferred material if cast iron is out of the equation. But to answer your question, the tub was probably never installed properly and the cracking noise is the bottom of the tub flexing. There's probably small voids beneath the tub bottom allowing for the movement.
Live with it.
Thirty years old, time for a new bathroom.
Live with it.
Thirty years old, time for a new bathroom.
