Losing a lot of water

I started digging up the closest pipe by the pool and by the pad and everything is pretty dry so I capped the returns just for fun and the video linked is the result from the farthest one from the equipment ( closest to the bottom in the pic in my previous post). It sounds like little rocks hitting the steel panels below the deck but I do have a sand filter. Could someone confirm that this is or isn't what the sand would sound like?
I also included a better sounding audio file.

https://dl.dropbox.com/u/37516213/Pool2audio.mp3
https://dl.dropbox.com/u/37516213/pool2.mov

Thanks,
 
So guys, has anyone ever heard this noise before?

Logic would say that if it's a leak in the line, it's probably quite close to the return fitting to be moving rocks around close enough to hit against the steel wall It it is that and not sand from the filter.

Does this make sense?
 
Hi guys, thanks for the quick replies. What you hear is directly from an iPhone recording it all. I'll post a better link with audio/video ASAP. To me it really sounds like little rocks hitting the wall and cement decking but I'm so new at this that I needed a bit of confirmation.

Thank you
 
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