It is a relatively new Athearn Ac4400 and it keeps making an annoying buzzing sound. The model runs perfectly fine at low speeds but usually when it make it faster it speeds up normally without any buzz then suddenly slows down and starts buzzing. Every once in a while I can make it run at a faster speed with no buzzing but it isn't very often. I've tried removing the shell, but it still makes the sound. Does anyone know how to fix this? I'm using a lionel controller with dc power and all my other locomotives run fine.