When I first start to accelerate I here this really strange noise like water being thrown around. It sounds like it's just right behind the firewall. It doesn't make the noise when I just rev the engine, only when I actually accelerate. What could this be?

 

Have you checked the coolant level lately?

 

Its just your coolant running in and out of your heater core. Thats pretty normal... My blazer does it, my dad's mountaineer, and my buddy's 1500

 

Coolant as in radiator or a/c?

 

Coolant as in engine coolant. Sounds like the cooling system has to be burped. Jack up the PS front corner, remove the rad cap & run it for a bit while keeping it topped up. have the heater on & rev it a bit every so often. Or take it to a shop that can hook up a coolant evacuation system & forceably burp the cooling system.