2012 Scion tC noise coming from rear. Sounds like water or rocks moving

What is your question?

2012 Scion tC with 105,000 miles on it. Just noticed that every time is rains, I hear a noise coming from the rear end of the car. It sounds like water sloshing back and fourth every time I hit a bump or when I drive but only at low speeds. I'm not a car guy so this could be something completely different than what I'm describing just not tryna get dicked over by a mechanic charging me a bunch to tear this thing apart. I have looked in the trunk and under the spare tire, there are NO leaks anywhere. Might sound stupid, it sounds like water is stuck in the rear bumper somewhere. The sound typically goes away whenever it's not wet out anymore but now the noise is there every time I drive so maybe it's not just water. Please let me know if you have any ideas, thanks in advance!

What makes this problem better or worse?

When it rains or is wet outside

How long has your 2012 Scion tC had this problem?

Every time it rains