Droning noise

I've got a 2001 Ford Focus 1.6 with just over 30K miles on the clock. At 70mph - 75mph i'm getting a very loud droning noise which appears to be comming from the front of the car. If i'm 1mph either above or below this
speed the noise disapears. Also on this certain road i'm getting a slow vibration comming through the car, almost like something is resonating against the floor pan. This seems to appear at any speed but only in this one road. I think it's to do with it having slight dipps in the road.
The car only went in for service last week so everything has been checked (although i didn't tell them about the problem). I've also jacked the car up to see if there is any play in the wheel bearings, but they appear to be fine.
The noise is still there if i put the clutch in, so i assume it can't be the gearbox? As such i'm starting to think that it's the driveshaft, but of course this was checked in the service. I have to take the car back to the dealer next week as my power steering pump is leaking, but i'd like to know if anyone has any ideas what it could be before that. I can imagine them replacing loads of parts and still not solving the problem.
Thanks.
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I would expect (in this order):
A wheel out of balance A tyre defect (bulge) Something loose in the engine bay - i.e. the engine cover or floor pan (if you have them)
It could even be the front bumper or grille resonating - it only take one loose screw/bolt and you would be amazed how bad this can sound at a certain speed.

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