Like many things on the diesel engine, the transmission cooler lines are subject to much vibration. The clamps that support them along each side of the engine pan have plastic inserts that prevent chaffing. Over time these plastic inserts break out and all you have left is a metal clamp rubbing against the aluminum tubing.
Problem & Solution
Common among these chassis:
On a number of occasions I have seen these clamps wear right through the tubing thus depositing your transmission fluid on the road as you drive. Check your lines soon. A good preventative repair is to take the clamps off and cut 8 mm rubber fuel line to make new soft inserts. Slit the short pieces of hose lengthwise to fit them over the tubing. Re-clamp the lines and you will not have to worry about them again.
While you are checking the metal lines consider changing the rubber flex hoses in the front. These are a major source of leaks. Also check for leaks where the lines attach to the transmission. You may need to replace the banjo washers there.