[VeeForty Deetoo]

some cars have an access panel under the rear seat for the fuel sender assembly, it's possible this is loose/missing/not fitted correctly? theres a release tab on the edges of the seat base to press in
Parking assistance, camera, rear - V40 2013 D4 Automatic - Volvo Cars Accessories image 28, credit to a previous contributor

 

[VeeForty Deetoo]

This may not be of any help but i had something similar on one of my vans and on a mates car.
Basically the sill drains were blocked,
The amount of water that came out was unbelievable but it cured the sloshing sounds.
Good luck.

Years back I worked at a used car sales garage, a Peugeot 309 had that issue, workshop trainee drilled a10mm hole each side and jacked the back end up, problem solved