I have 2004 Monte Carlo with the 3.4L and I am thinking that i have an antifreeze leak. I noticed a sloshing sound when starting the vehicle up and while driving so I checked my coolant level, thinking that it may be low (temp gauge was always normal). I waited until the next day to check it when it was cold and the reservoir was completely empty! So I filled it up and checked it again the next morning, and it was a little low so I topped it off. I checked the radiator and topped it off as well.

Side note: Less than a year ago I did a partial drain and refill so this could have something to do from back then I suppose if I didn't put enough in it then or something

 

Your coolant elbows may be the plastic ones and may be cracked and leaking. Many members have replaced them with the metal ones. just an idea

 

I have had my coolant elbows leak like Barbara said. If the 3.4 are the same as the 3.8 look closely at the tensioner pulley bracket, coolant flows through the bracket via 2 elbows that will eventually crack or leak. Replace with metal ones.

 

The 3400 does not have coolant elbows. When checking if you are loosing anti-freeze, pop the cap off the radiator and check and fill the radiator directly.

Best way to hunt down a leak is get a radiator pressure test from the parts store, with the car cold and off, pressurize the system and look/listen for leaks. ON a 3400 could be a hose, water pump or LIM gaskets are the common culprits.

 

I found out that it was the water pump so I put a new one in and also changed the upper and lower radiator hoses. I'm crossing my fingers that there is no more leak! Thanks for the help everyone!

 

Glad you got it fixed!