Land Rover Repair: Over-heating Discovery Series II, cylinder head gaskets, discovery series ii

Question
I have a '00 discovery series II that has begun to start over-heating once the engine warms up. I've replaced the coolant/radiator fluid but it still heats up. It's only boiled over once but that was prior to my replacing the fluid. I don't have leaks either (weird). In addition, after the engine gets warm it sounds (and feels) like a small animal is trying to get out of my engine - thudding and the like which makes me wonder if my air flow for the engine is blocked and its trying to correct itself. The car hasn't had a problem like this before (only has 76K miles for a '00 model) and I'm hoping it's not going to cost an arm and a leg to get repaired.

Answer
The noise you have described, the one that sounds like a small animal trying to climb out, sounds like a symptom of severe overheating.  I would have the car block tested to check for blown cylinder head gaskets and/or block problems.  These engine suffer severely from slipped or porous liners.  If the vehicle passes the block test, I would immediately replace the thermostat and radiator.