Hi Neil,

There are several items that can cause the sound you describe. I have listed what I can think of off the top of my head below.

I believe you have a bad bearing somewhere. The possibilities are;

1) A wheel bearing (most likely one of the front wheels).
2) Bad CV (Constant Velocity) Joint (again most likely one of the front wheels)
3) Bevel Gear in the transfer-case or some call it the “Angle Gear”.
4) There is a coupling located at the front of the rear drive shaft that can be bad

What I have listed are the common issues XC owners have had. Items 1,2 and 4 are easily identified and repaired. Item #3 is going to be a real pain in your wallet.

Also a sticky or stuck brake caliber can cause a howling noise at high speed but not at low speeds. Again this is an easy problem to find and fix.

Good Luck,

Steven---