Vauxhall Vivaro body filled with water

You should not really have water in your panels,some cars have small drain holes at front and rear of the sill,s and of course there,s the usual front scuttle panel drain ,s that are situated on the bulkhead,check they are clear.

Also check door drains at bottom are clear .

Remove the torx screws and plastic step cover next to the bottom of the rubber door seal. I found a lot of water there in the seam just above the cill - mopped it up the first time, but next time drilled a 6mm hole straight down through the bottom of the cill and hey presto, no more sloshing noises!