"HIGH" pitched ringging heard only when weather is cold outside

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this sounds weird, but when it is cold outside ie. 0 to 40 degrees,there is a high pitched ringging heard at different times - not constantly, the ringging pitch can be raised or lowered by lightly tapping throttle pedal or say you are driving with overdrive off and decide to turn it on, you can hear the shift in transmission through the ringging,this ringging again, comes on at no real particular speed, but when it does start,you can hold the ringging sound by "playing" with the throttle pedal, is this the transmission pump or maybe torque converter? when the weather gets warm outside,you don't hear the ring at all,only when it's cold. Truck does have 185,000 miles on it.