When towing a heavy load, 13,000 pounds, up a long hill, I sometimes get a whistling sound coming from the engine at the same time the auxillary fan/pump comes on. It doesn't occur all the time, but more often on the more severe hills. Today it happened on the Lewiston, ID, hill; 6 miles, 7% or more grade. If I let up briefly on the accellerator the noise goes away for 5 to 10 seconds.
Anybody else experience this?