Hoping someone can help. Can't figure this site out to send a new post. Just had new kitchen tile floor installed. They are 12x24 porcelain tile. Occasionally when a certain tile is stepped on it makes a popping noise. Does not happen every time. There is no visible crack to tile or grout. Can anybody help me with a cause for this??

So in the process of removing my old tile there seems to be some holes/cracks in the seem between two slabs that run the length of my condo from one end to the other through the dining room kitchen and foyer. The worst parts seem to be in the dining room where there about three or four big holes about half inch wide by 3 to 4 inches long. The holes go all the way through the subfloor and I'm pretty sure there's only empty space and metal studs/drywall separating our floor to the ceiling of the unit below so I'm trying to figure out how to fill that in without having any filler concrete/cement seep through to the ceiling of the neighbor below? In doing some searching I found a foam backer tube that is used for concrete expansion joints that I think would do the trick to fill in the holes by stuffing it in to fill the bottom of the holes which I think would allow me to fill in the top half with cement. That actually brings me to my next question. Would I need something more flexible to fill in the gaps/holes, or would I need a cement/concrete patch type of filler to do the trick to avoid future cracks? Could I just use the mortar I'm planning to use for heavy tiles which I'm told is a little more flexible? Or some form of cement patching compound is need before the mortar is applied with the tiles?[ATTACH]78277[/ATTACH]