Sometimes an armature with a few shorted windings will develop a hammering type noise caused by internal stresses. Look closely at the commutator where the brushes ride. If the leading edge of one or more commutator bars shows deterioration or burning and the brushes have a ragged looking edge, that's a pretty good indicator of winding problems. Back when we rebuilt our own starters and DC generators, just about every good shop had a "growler" for testing armature windings. I've still got one stashed away somewhere, but I haven't used it in years.
You can spin-test a sealed bearing by holding it against a wire wheel buffer on a bench grinder.
Jerry
