It sounds like one of the magnets came unglued from your motor shell. I have had that happen on two DC motors in the past, although neither were for the Elec-Trak. One had a broken magnet which I fixed by gluing the magnet pieces as close together as possible.
Nope, all four magnets are nice and snug in there.I like the solution for broken magnets though. Also when putting the armature back in be *SURE* to hold it. It will go screaming towards the magnets, and I'm sure it could break them.
Chris