Truck is a 2003 Explorer, 4WD. This is driving me nuts. Only during the winter months, when cruising around I hear this faint, high pitched squeal (almost like old time radio engine wine) from under the dash. Its realted to the throttle because if I mash the gas or let off, it goes away. But under constant throttle load, even if using cruise it is persistent. I never hear it during the warmer months. And nothing to do with the radio or electronic HVAC.

Anyone ever experience this gremlin or have any idea where to start. I drive 2500 miles a month for work and its highly annoying. I crank the radio when by myself, but with customers that is not an option.

 

Is the noise there when the engine is running and the truck is not moving, say in neutral or park?

 

No...only when under load in drive. Seems to be related to rpms in the 2500 range

 

This will seem strange but, the only thing that keeps coming to mind is a possible small vacuum leak at a heater control or in one of the vacuum hoses. I suggest that you get a vacuum gage and see what the reading is at the time that you hear it start and again when you pass out of the sound, then in an open space, set the parking brake and block the wheels and try to duplicate the vacuum readings at a standstill by having the truck running and in gear, if you can duplicate the sound you will need to get someone to feel around under the dash until they touch the offending hose or componant causing the noise to change or go away. I had a similar problem under the hood of my Mark VIII a few years ago and it was a tough one to find. In my case it was a molded hose that was collapsing at a certain vacuum reading and eventually cracked enough to make the irritating noise.
Good luck with it and let us know what the fix is.