If that engine has EOB (Engineer on Board) cruise control, that is probably the culprit. Turn the cruise feature off and run the engine again. If it doesn't squeal with cruise off, then the high frequency pulsing of current to the motors is causing them to squeal. Adjusting the pulses (see the EOB manual) can reduce, but not eliminate, the squeal. Some or most engines do it to some degree, but there are also some EOB engines that are quiet.