Stuart,
	If the noise is speed related, but not engine speed related,
then its the chassis. If you can get it at a lower speed in a lower
gear, then its the engine. Easy way to tell? Accelerate to the range
where you hear the noise, get a bit faster and then put in the clutch
and let the gas pedal go. The engine rpm should drop, and if the problem
is an engine problem, the sound should go away. If its a chassis problem,
it will persist, fading as your speed falls. 
	I had a vibration at 70 in my Rabbit, and it turned out to be
a tire. I'd worn out the rears completely, and had exposed a spot of
the steel belt in one place, and the tire's vibration energy overwhelmed
the shock-absorber at 70. New tires fixed it, pronto!

	Bill