Could it be the coupler(s) firing inappropriately? This can happen if there's a short in the coupler wires that only occurs when the engine goes around a curve. I've had that happen before. In my case, the culprit was a coupler wire where the insulation had gotten peeled off and the bare wire was exposed. When the engine rounded a curve, the bare wire touched the metal frame and coupler would start buzzing.
-Eric Siegel
