Southern California is getting battered by rain. Last night I noticed a water sloshing noise when the car accelerates from a stop or came to a stop. It sounds like it's coming from the floor. I popped the rear passenger side carpet but it was dry. I checked the door drains but they were all clear. I also pulled that weird plastic plug out of the floor boards on the passenger front side but found nothing.

Where else could it be?

Is there a hollow in the running boards (like where the Frog hid the heroin in the French Connection)?