I had the water sound go on for a while, then it went away. Truck started to over heat so i had to add coolant. My radiator was all filled with dirt and stufff that doesnt belong in a radiator. Did the whole flushing of the system and my mechaninc said not to use dex-cool antifreeze. I was just wondering what that water noise is?

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Since you had low antifireeze it made bubbles and that was the noise which was coming from your heater core.

 

The sound was air in the antifreeze. Dex-cool does not tolerate air, and forms a sludge substance that you saw in your radiator. It will also rust the engine, and you should also see the rust in the overflow. The usual GM fix at this point is a radiator acid flush. Google dexcool and GM and blazer and you can see a pile of information on this. the GM service bulletin for the acid flush is also posted somewhere on Yahoo.