Well, your fuel filler neck is on your passenger side but the gas tank is under your entire rear seat. Think of it as a kidney or a bean allowing the driveshaft to go under its middle. All this to say, if the tank was sloshing you should hear it on both right and left sides.
Have you had a lot of rain in your area during this period?
Can you pinpoint whereabouts the sloshing is coming from? (You might need to sit in the back while someone else drives).
If it's
overhead, then it might be some water in the sunroof assembly, although if it is enough volume to slosh around, it should also get your headliner and/or C-Pillar trim wet. Feel around for dampness.
If it's coming from a little
lower, then I'd check these:
- Your passenger doors. If the drains are clogged and the weatherstripping fails, the door might hold water.
Just open and shut each door to see.
- The rear quarter panel. This has to be accessed from the trunk and you remove some trunk liner clips to see if water is accumulated there.
So those area you're looking at is directly behind the rear wheel, above the bumper cover.
- While in the trunk area, you might as well check the spare wheel compartment.
This is easy and more likely than #1 or 2 above.