I have a 2001 GMC Sonoma 6 cyl. I can hear fluid moving behind the dash when i accelerate. The more i push the gas, more fluid i can hear moving. My heater core was replaced about 3 months ago. I could hear the fluid before the replacement, and i still hear it now, however, I do not smell antifreeze or get any residue on my windshield like i did prior to the replacement, so i do not think there is a leak. What else could this be? Thanks in advance.
Probably leftover fluid from the previous heater core leak.
There’s probably air trapped in the cooling system and what you’re hearing is the air/coolant passing thru the heater core.
To remove any trapped air from the cooling system, get the engine up to operating temperature. With the engine idling, slightly loosen the upper radiator hose clamp. Take a flat bladed screwdriver and slip it between the the upper radiator hose and hose neck on the radiator. Allow the engine to idle until all that comes out of the radiator hose is coolant. Retighten the hose clamp.
Tester
Do what tester says. If you still have a problem, do what tester says while facing the truck up a steep hill.