Last week I purchased a DCC on board GP7.  The engine lights up and runs but buzzes loudly, especially at low speed and won't hold steady speed.  I am powering my layout with a Digitrax zypher extra.  Also, I have to turn my throttle up to 1.5 before it even moves.  I this how these engines run?  Is there something I can do to get it run better?  I took it apart and didn't find anything major wrong with it.