The Curtis controller will emit a tone and reduce power in an overheat condition. My 1231C did this when the controller's cooling fan leads accidently became disconnected. If I backed off on the throttle for just a few seconds, it would cool down, resume power, and the tone would go away.
Here's an excerpt from the Curtis manual:
Controller operation is normally silent, with three exceptions: (1) A 1 kHz tone may be heard during plug braking. This noise is normal and indicates that plugging is taking place. The noise will stop when plug braking stops. (2) The same noise may indicate overtemperature. The controller shifts frequency during overtemperature from its normal 15 kHz to 1 kHz (1.5 kHz on “C” controllers), providing an audible tone to alert the operator to the overtemperature condition. (3) The frequency shifting feature on “C” controllers produces a 1.5 kHz tone during the first 15% duty cycle of the PWM output. This tone may be heard during low throttle, slow speed maneuvering.
If the controller gets hot, it does not necessarily indicate a serious problem. Curtis PMC 1209B, 1221B, and 1221C controllers protect themselves by reducing power to the motor if their internal temperature exceeds 75 ｰC (167ｰF). The 1231C controller begins reducing power at 85ｰC (185ｰF). Power output will be reduced for as long as the overheat condition remains, and full power will return when the unit cools.
It sounds like you may be running near the limit on overheating. I'd consider adding additional controller cooling with either more heatsink or cooling fan capacity.