Nissan Repair: 1994 Nissan Quest, tiny explosions, radiator cap

Question
I turn on the van and it is fine until the engine heats up then it starts doing like tiny explosions on the engine. Not literally an explosion but a loud sound like if the timing is off on the engine. They checked that and they tell me that the fine. What could it be? I turn it off then it cools down and when i turn it on again the same thing, works then engine heats not over heats just heats and the same thing. Please help, i really cant afford anything else.

Answer
Eliseo,

I would replace the thermostat and change the antifreeze with new 50/50 mix with demineralized water.  Also, replace the radiator cap.  Sounds like the engine is on the edge of overheating with out overheating.  The corrections I mention are normal for correcting the problem.