If you have some trash rolling around in the bottom of the tank, it should be no surprise that it would settle somewhere other than the where the fuel exits the bottom of the tank from time to time.
Happened to me more than a couple of times. Most common was big flakes sluffing off of the side of the tank. I believe that ethanol had a part in that. A new tank solved that problem.
I converted three AD 3100 trucks to the later sending units which pulls the fuel from the top of the sending unit instead of bottom of the tank.
