I have acquired a Bachmann GE 70 Ton DCC switcher.  It runs poorly.  I've disassembled it to the point where the motor is only turning its flywheel --both fore and aft drive shafts disconnected.  Using my NCE controller, as I increase speed, the motor just hums/vibrates loudly for the first two-three speed notches and then begins to turn around the fourth speed notch --and continues loudly.  Is this a DCC decoder issue or a motor issue?  It is really unusable in its present state --runs like a cheap, bad toy and not a representative model.