Unfortunately there are still many possibilities.
I assume the only specific condition for this sound is it occurs, regularly with an even tone, at any time over 40 mphs. I assume this is independent of car temp, throttle position, outdoor temp, RPMs, ect.
This being the case, it very possibly is not related to any engine components. This essentially leaves the suspension or drivetrain.
Shut your engine down while this is occuring, and observe the status of the noise. Does it dissappear? If not, you may have a bad wheel bearing.
If it does disappear, it could be related to the clutch or transmission. Do you have a 5 speed? Push your clutch in while this happens. Does it go away? If it doesn't, put your transmission in nuetral and release the clutch pedal. How about now?
If you've an automatic, try using neutral.
If it persists after disengauging clutch/transmission, but dissapears with shutdown, it is very likely a component related to your engine.