Ford Escape four wheel drive models may develop an abnormal noise from the transfer case. This may sound like a bearing howling first, but will turn into a clicking noise at higher speeds.

The transfer case on these models is notoriously troublesome, and can become damage from normal use, and the sound of the failure is notably close to the noise heard from mud tires on pavement. 

To eliminate the noise, the vehicle should be inspected, and, if the transfer case has suffered internal damage, it must be replaced, not rebuilt. 

Changing the transfer case fluid on schedule may help, but many failures are before the first fluid change interval.