If it's by the water pump and not up in a cabinet, its probably a loose fitting, either PEX or vinyl. You need to find out exactly where the leak is and you may just be able to fix it. If it's PEX you will need to go get a crimping tool and some bands (Home Depot) at the very least if you want to DIY.
If it is in a cabinet, then it could be the vacuum breaker which you can pick up a replacement and just screw in the lines and replace it.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME