This may not be exactly what you're experiencing, but I've noticed this with my 99 Bravada.
I sloshing may just be the gas tank - I've noticed it with mine when it gets below 1/2 tank. Especially with the radio off and the windows rolled up.
Do you live in a warm/humid area? I've noticed in the summer when I use the AC a lot that sometimes the evaporator drain tube will plug up and start to give a moldy/musty smell. If it gets really plugged (it only did this to me once), you might hear a sloshing noise coming from the passenger side floor area. Typically when that happens you'll have wet passenger side carpets when the evaporator box starts to leak into the passenger compartment.
Luckily (or unluckily, depending on how you look at things) in Michigan we only have these problems about 2 months out of the year....
