Well, if a main pump was $20 instead of $130 or so, that would be enough for me, I suppose-- that is, all those symptoms disappeared when you replaced it. Otherwise I'd want to prove it had the electricity it needs (at its terminals) and the fuel it needs to run quietly, at its input.

However, 18V is about 30% more than it needs, and though that in itself won't make it noisy, it will generate more heat, and if the fuel is low in the tank, or the tank pump inoperative, the resulting heat, given time, will create fuel vapor and that will make it noisy.

I still think your trouble there might have been in the ashtray, as I posted in your thread. That's another topic though -- you need to move your attention from the computer screen to the 740 and get that voltage down before your battery and everything else is toast.
--
Art Benstein near Baltimore

If lawyers are disbarred and clergymen defrocked, doesn't it follow that electricians can be delighted, musicians denoted, cowboys deranged, models deposed, tree surgeons debarked, and dry cleaners depressed?