mll, This one's a long shot, and for the end of the list, not the beginning. A cracked crankshaft will make exactly the noise you are describing. This will be most common at rod pin number 4, sometimes number 3. Usually, when it has gotten that bad, you will be able to see it with the naked eye - but the crank obviously needs to be out of the engine. Sometimes you might have to magnaflux it in order to detect it, but one thing is sure: if it is a cranked crank, it is only a matter of time until it lets go...
Good Luck with this one!
Chris