Have the hub bearings been checked as nwm suggested?
It's noticeable at higher speeds (as you're suggesting) and has a definite vibration but also noisy. In my case, the noise increased/decreased when the wheel was turned (even just slightly) when around 70mph.
One VERY easy way to check this:
Find some space (car park is good) where you can do some slow full circle turns.
Put the steering on full lock (any direction)
Drive slowly - listen for a rhythmic clunking noise
Hear anything?
If not, put the opposite lock on and listen again?
If you can hear a clunking noise, it's highly likely that one of the bearings has gone, as I said, I've had BOTH go!!!
Also - CV joint could be gone, very similar symptoms to a hub bearing apart from this will get so bad that you won't be able to ignore it. Saying that, the bearing on mine appeared almost out of nowhere and got progressively worse over just a few days to the point where it had to be sorted.
Hope this helps?