My 99 has been leaking coolant for some time, sporadically leaves a puddle under the right rear of engine. Removed the pans this weekend and a coolant stain is very visible on the engine block, the starter blocks the view but it appears it is running down the block from somewhere behind the starter. No visible signs of leakage up top from the lines?

Any suggestions?

 

Heater hoses?

Go squeeze the upper rad hose when the truck is cold, listen for air bubbles coming from the hoses. It works for me to find coolant leaks.