2004 XK8 58000 miles
After 4 hours of sometimes spirited driving in the Canadian Rockies, the car developed a very mild, hollow sounding howling noise. This sound was present and persistent only at the specific speeds of 35-40 mph and 55-65 mph. When present it did not change in any characteristic with throttle changes (load), nor with any manual gear change (engine rpm/transmission gear), nor with the application of brakes, nor with the lateral stress of significant cornering forces. The pavement characteristics had also changed a number of times and the sound remained consistent (tire tread)..
The wheels were not found to be hot as might be expected from a failing wheel bearing or dragging brake. Engine instruments (such as they are) were nominal. Fluids levels that are accessible are at normal levels.
The car had been driven for a period of 3 hours without this sound presenting. After a short period of rest for refueling, etc of about 10 minutes the sound became present. It persisted as noted above until a sceond stop about 30 minutes further along, at which time I performed the checks of wheel temp and tire pressure and found no anomalies. This stop lasted 15-20 minutes while libations were purchased.
Upon resuming travel I noted that the speed ar which the sound occurred had changed. It only occurred at 40-50 mph now and was not present at any other speed up to 90 mph. After traveling a further 15-25 minutes the noise gradually faded and then disappeared. The precise time here is not known as road speed through the twisting mountain roads, in traffic, were quite variable.
I will be resuming travel tomorrow, and wonder if there is something else that I might seek out for attention today.

 

Check transmission oil level ..mine growled as the gbox was only half full