basement tub

Im finishing off my basement. About a month ago i set my tub in a mortar bed then put in several bags of sand and let sit for two days. I then installed three piece surround. The tub feels real solid, but i noticed today when i stepped in it. there is a slight popping under floor when i shift my weight. I hope this is acceptable. i would hate to tear all this out. Will this be okay. By the way, it is a acrylic tub. thanks

I would have to say that you probably should not have placed the sand in the tub. The acrylic has a bit of flex, transferring to the wet mortar, displacing the mortar. I suspect that the tub has a ledger board that you installed along the edge. It may be hard to determine where the popping noise is coming from , the bottom flexing or the edge around the tub flexing.

It is the floor flexing I can feel it. Do you think it will be a problem

A problem? Thats a relative term
A nuisance for sure. It may create future issues (I don't want to say "it will") but always a possibility.. At this stage, other than starting over, I don't know what else to suggest. Spray foam is OUT OF THE QUESTION.Don't even think about it. It will be hard to control the expansion.

If you can live with the popping for a while and see what happens, It may last for years without any other issues

You put sand in the tub which pushed the floor down. Then you took the sand out, but since the tub was NOT glued in place to the mortar, the tub's floor "rebounded" to its original position which was above the mortar. Now when you get into the tub, you are feeling and hearing the tub settling down onto the mortar. If you were to fill the tub with water first, that would simulate the sand you used and the tub would not make any noise. When a weight is suggested for a tub, it is because the user plans to use an expanding foam, and that could LIFT the tub without the ballast, and the foam DOES stick to the tub and lock it in place.