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.
was that bathroom ever remodeled? If this was a recent install, I would suggest more investigation as it does sound like there is an installation problem that should be remedied. If it is original, I would go along with the other poster.
I have mentioned previously on this forum that this is a common problem I find in my "HandyMan" business, and there is a solution rather than pulling the tub/shower unit. This involves shooting something like "Great Foam" between the tub/shower unit's bottom and the flooring IF you can gain access to this area. This is not always easy, but a do-able project. Your daughter has now invested monies into this home and has to decide what to do before a crack develops that will leak water.
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