thanks for all the help guys, @tkeyyy I'm not sure how hard it was raining that night, but when I went
out to check on my car it was dry and I couldn't find any sign of water in my car or in the tray but when my
car is rolling from a stop I can hear some water? not sure but sounds like something sloshing around behind me.
Only other explanation would be that I'm hearing the gas in the tank? However that idea sounds unlikely.

 

yea you shouldn't be able to hear the gas in your tank. haha.
any pics of the crack?

 

I swear I hear gas in my tank in the right conditions. Maybe it's all in my head.