Weird braking noise

Hi everyone,

I have just bought a 2000 celica gt, and I have driven it around. However I notice this weird sound inside the cabin coming from the rear almost like its coming from beneath the rear passenger seats. The sound only happens when I am braking and sound like water swooshing in a plastic container. Any ideas what it might be? Could it possibly be my rear brake drums? or my gas container? maybe there actually is some water stuck some where and need to drain it?