Based on your description, I agree with the thermostat. If the oil pump was bad, the light would be on all of the time. When the engine gets hot (overheating) the engine will make noise. If you are loosing coolant, that could be just from overheating. You can check the waterpump easily. First, when the bearing goes bad in them, they are noisey. Also, there is a weep hole (aprox 1/4" in diameter) under it that coolant will begin to leak from. Check that to see if there is evidence of it leaking.
Start with the cooling system. A t-stat is cheap and I would flush the system too. To me, it sounds like the problem doesn't get bad until the engine begins to overheat, which will thin the oil and could cause the oil light to come on.
If you have other questions, let me know.
Tuesday, September 22nd, 2009 AT 10:00 PM