Sloshing under dash

[Nofuze]

Hey People, new to this forum...hope someone can help out!
I have a '97 Jimmy that has a very annoying "sloshing of water" sound coming from behind the dashboard. Everytime I move the truck, ie.turn, accelerate etc. it sounds like a gallon of water sloshing around. The mechanic first suspected a plugged A/C drain, not allowing condensation to escape, but I suspect something else. My heat is luke warm at best until the truck has been running for an hour or so, then starts to get hotter. I've had the core flushed several times which temporarily improves the situation and I find my coolant level down about a litre or so. I suspect the heater is leaking coolant and getting trapped in behind the dash somewhere but it never leaks into the passenger compartment or on the road.
Any thoughts on how and where this may be occurring?

Thanx.

[BlazerLT]

1.) Go get your cooling system and your heater core reverse flushed with new coolant.

2,) Install a new NON ACDELCO rad cap. Don't install a new AcDelco, they are flawed.