Try finding roads with a varied road surface. If the noise goes away on very smooth roads, it;s probably a wheel bearing, or tire. Otherwise, anything that can move or rotate, could be suspect. Try braking slightly at speed to see if that changes the sound, if the brakes are sticking or there is a rusty rotor, this could cause a noise.
If the bike has been idle for a long time, this could have caused the belt to take a set. This is a good place to look after other easier checks.