Unwanted high pitched sound

I have a 2005 Merc Mountaineer. When traveling at a constant speed (important that it is constant) I get a high pitched sound that reminds you of that high pitched tone you hear when taking a hearing test. If I slow down or if I speed up a mph or two, it goes away --- only briefly though. Once it settles into the new speed, the high pitched sound returns. I thought at first it was my cruise control, but if I shut the cruise control off and hold my foot on the gas as evenly as possible, the high pitched sound is still there.
It is about to drive me more crazy than I already am. Any ideas would surely be appreciated and I will ad your name to my Christmas card list :)
Thanks so much....
Jack

Welcome, fellow Iowan!

My initial suspect would be either a rear pinion bearing or a rear wheel bearing. Rear wheel bearings are pretty common, rear pinion bearings not as much. You could try changing the rear differential fluid and see if that changes the sound. If so, then it's probably pinion bearing.

-Rod

Welcome, fellow Iowan!

My initial suspect would be either a rear pinion bearing or a rear wheel bearing. Rear wheel bearings are pretty common, rear pinion bearings not as much. You could try changing the rear differential fluid and see if that changes the sound. If so, then it's probably pinion bearing.

-Rod

Thank your for the welcome Rod. And thanks a great deal for the info. I will begin by having the fluid in the differential changed and see what happens. I always seem to get strange and odd ball problems.
Your reply was more than appreciated...