What kind of Water/Antifreeze for Aluminum
The distilled to drinking water switch wont make any difference except that you might have side reactions with the ions taking place in the drinking water. distilled is the best choice (if you have it) because you dont have any other chemicals in the water to react with metals, or metal ions in solution. Also, if your using well, city, etc, drinking water, you have a lot of iron (as ions) in the water. These ions will want to oxidize the aluminum (for chemical reasons I wont go into) and eat away your aluminum. Also, with drinking water, you make your cooling system fluid electrolytic, which means if the engine block has a better ground through the coolant to the radiator mount than the ground strap on the motor, you'll form your "pellets" there (probably iron pellets, see if their magnetic) if they are, you've got a bad ground to the block.
I may have spouted off alot for nothing, but yeah, check your engine ground.