04 2500hd pouring coolant from somewhere. Looks like lower back if the motor but can't tell for sure. Under 100k

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It is possibly the EGR cooling pipe that runs to the EGR cooler there is 2 of them 1 on each side I would check them first and also check the heater core pipes if you trace them back to the engine you should be able to find the leak they are both hard to see as there at the rear of the engine but if you use a small mirror it helps.

It is possible but not as common as the EGR and if it is on the drivers rear side it can be the turbo cooler pipe.

Wait till it is cool the top of the coolant with water put the cap on and run the vehicle until it is warm then check and look around the back of the engine until you see the leak if it does not leak when running stationary you  will need to get a cooling pressure test done.