Delta Shower Pan Crack/Pop Noise

Installed this pan over a week ago, maybe two. I've been busy so I haven't been working on this project until the other day.

There is one spot in my Delta Classic shower pan that makes a pop/crack noise when pressure is released. This did not occur the day of installation.

Pan is level, floor is level. I used Dap as adhesive underneath per Delta's recommendation. Drain is of course installed and the base has been screwed into studs. This spot is about 7 inches in from the open side of the pan.

Here is the video with sound:
http://imgur.com/gallery/7CE4vh8

 

I am not familiar with Delta pans, but I know Dreamline says to use thinset to set their pans down. I find it odd that Delta says to just use DAP.

At this point, I would try to spray some expanding foam from underneath to fill that void if you can reach in there . I would get a long flexible tube and try to route it under the pan to that spot and spray the foam liberally. Wait about a day before putting weight on it.

 

This is why I either install a lead pan 4 PSF sheet lead or a Terrazzo base as it lasts and is noise proof and never used any other materials except once I made a pan out of copper for a special application