The "plumber" who installed our fiberglass tub did not setted the tub to the floor, now when we stepped into the tub it makes a crackling noise, how can we fix it without removing the tub?

Thanks
Lino

What brand is the tub? Is it perhaps a Kohler Sterling.

Please answer the question. Some brands require a setting material while others do not. If your's did require a setting and it was not done you have recourse with that plumber.