1997 Mercury Marquis Spark knock?

I have a beautiful 97 Grand Marquis, but has the dumbest of problems. Well I say that cause its been wooping me for weeks. The trick about my car is its inherited, so I know its entire history. About 100,000 miles ago it had a brand new engine put in it, cooling intake faliure of some sort. The tranny however is totally original, but has been outstanding. Anyway After I got the car I drove about 10,000 miles and started noticing a very fast rattle only when the car is underload. Manly in acceleration or hill climbing. ALSO only when the engine is warm. The problem got worse over a period of weeks, so I parked it to figure this out. I have put fresh plugs in it, checked injectors, tried higher octane, checked fuel pressure, replaced a couple of the many o2 sensors, even let the Ford house run some miracle carbon cleaner through it, and worked on the cooling system (new thermastat rated 10 degress less than the original.)STILL THE SAME. My question to you is. Is this spark knock. Or just PLEASE I NEED NEW IDEAS!Ps. It does great in cool weather, but summer is a nightmare on it.